NATURAL  PHILOSOPHY: 


EMBRACING 


THE   MOST   RECENT   DISCOVERIES 


VARIOUS  BRANCHES  'OF  PHYSICS, 

*W    ' 

AND  EXHIBITING    ^^  ^J^\  -/*     -j 

f    i 

THE  APPLICATION  OF  SCIENTIFIC  PRINCIPLES  IN  EVEEY-DAY  LIFE. 


ADAPTED  TO  USE  WITH  OR   WITHOUT  APPARATUS,  AND  ACCOMPANIED  WITH 

FULL  DESCRIPTIONS  OF  EXPERIMENTS,  PRACTICAL  EXERCISES, 

AND  NUMEROUS  ILLUSTRATIONS. 


BY  G.  P.  9UACKEOOS,   LL.  D 


•  t 


PBINCIPAL  OP  "THE  COLLEGIATE  SCHOOL",  N.  Y. ;    AUTHOR  OP   "FIRST  LESSONS  IN 

COMPOSITION",   "ADVANCED  COURSE   OP  COMPOSITION  AND  RHETORIC" 

"  ILLUSTRATED  SCHOOL  HISTORY  OP  THE  UNITED  STATES  ",  ETC. 


NEW  YORK: 
D.    APPLETON    AND    COMPANY, 

90,   92  &   94  GRAND    STREET. 
1870. 


By  the  same  Author: 

PIRST  LESSONS  IN  COMPOSITION:  In  which  the  Principles  of  the  Art  are 
developed  in  connection  with  the  Principles  of  Grammar.  12mo,  pp.  182. 

ADVANCED  COURSE  OF  COMPOSITION  AND  RHETORIC :  A  Series  of  Prac- 
tical Lessons  on  the  Origin,  History,  and  Peculiarities  of  the  English  Language, 
Punctuation,  Taste,  Figures,  Style  and  its  Essential  Properties,  Criticism,  and  the 
various  Departments  of  Prose  and  Poetical  Composition.  12mo,  pp.  451. 

AN  ENGLISH  GRAMMAR:  12mo,  pp.  288. 

PRIMARY  HISTORY  OF  THE  UNITED  STATES:  Made  easy  and  interesting 
for  Beginners.  Child's  Quarto,  splendidly  illustrated,  pp.  192. 

ILLUSTRATED  SCHOOL  HISTORY  OF  THE  UNITED  STATES :  Embracing 
a  full  Account  of  the  Aborigines,  Biographical  Notices  of  Distinguished  Men, 
numerous  Maps,  Plans  of  Battle-fields,  and  Pictorial  Illustrations.  12mo,  pp. 


Entered,  according  to  Act  of  Congress,  in  the  year  1S59,  by 

G.  P.  QUACKENBOS, 

In  the  Clerk's  Office  of  the  District  Court  of  the  United  States,  for  the 
Southern  District  of  New  York. 


PREFACE. 


THE  importance  of  the  physical  sciences  is  now  so  generally  ad- 
mitted that  there  are  few  institutions  of  learning  in  which  they  are 
not  made  regular  branches  of  study.  And  very  properly, — for  what 
can  be  more  interesting  and  instructive,  what  more  worthy  of  the 
attention  of  intelligent  creatures,  what  more  calculated  to  inspire 
their  minds  with  a  thirst  for  further  knowledge,  and  fill  their  hearts 
with  reverent  gratitude  to  the  Divine  Being,  than  an  acquaintance 
with  the  laws  of  the  material  world,  the  mysterious  influences  con- 
stantly at  work  in  nature,  and  the  principles  by  which  atoms  and 
worlds  are  alike  controlled  ? 

It  is  in  the  hope  of  investing  this  subject  with  a  lively  interest, 
and  bringing  it  home  to  the  student  by  exhibiting  the  application 
of  scientific  principles  in  every-day  life,  that  the  Natural  Philosophy 
here  presented  to  the  public  has  been  prepared.  The  author  has 
sought  to  render  a  subject,  abstruse  in  some  of  its  connections,  easy 
of  comprehension,  by  treating  it  in  a  clear  style,  taking  its  princi- 
ples one  at  a  time  in  their  natural  order,  and  illustrating  them  fully 
with  the  facts  of  our  daily  experience.  The  range  of  topics  is  com- 
prehensive. By  avoiding  unnecessary  repetitions,  room  has  been 
found  for  chapters  on  Astronomy  and  Meteorology ;  one  of  which 
subjects,  at  least,  has  heretofore  been  invariably  omitted  in  similar 
treatises,  though  a  summary  of  both  is  important,  as  time  is  seldom 
found  for  pursuing  these  branches  in  separate  volumes. 

The  incorrectness  of  many  of  the  text-books  on  Natural  Philos- 
ophy has  been  a  subject  of  general  complaint.  Grave  errors,  both 
of  theory  and  fact,  have  been  handed  down  from  one  to  another,  and 
the  results  obtained  by  modern  research  have  been  too  often  over- 


4  PREFACE. 

looked.  In  preparing  this  volume,  every  effort  has  been  made  to 
ensure  accuracy,  the  most  recent  authorities  have  been  consulted, 
and  it  is  believed  that  a  faithful  view  is  presented  of  the  various 
sciences  embraced,  as  far  as  they  are  at  present  developed.  It  is 
the  intention  of  the  author  to  keep  his  book  up  to  the  times  by 
constant  revision,  and  to  make  such  alterations  and  additions  as  the 
progress  of  discovery  may  require. 

Two  styles  of  type  are  used  in  the  text ;  a  larger  size  for  lead- 
ing principles,  a  smaller  size  for  descriptions  of  apparatus  and  exper- 
iments, explanatory  illustrations,  &c.  By  confining  a  class  to  the 
former  when  the  saving  of  time  is  an  object,  a  brief  yet  complete 
course  may  be  taken.  Questions  at  the  bottom  of  each  page  will 
be  found  to  facilitate  the  examiner's  duty,  and  to  afford  the  pupil  a 
means  of  testing  his  preparation  before  reciting.  At  the  end  of 
such  chapters  as  admit  of  it,  easy  practical  examples  have  been  in- 
troduced, to  illustrate  the  rules  and  principles  set  forth. 

An  important  feature  of  this  work  is  its  adaptation  to  use  with 
or  without  apparatus.  The  majority  of  schools  have  few  facilities 
for  experimental  illustration.  The  wants  of  these  are  here  met  by 
a  free  use  of  engravings,  full  descriptions  of  experiments,  and  expla- 
nations of  then*  results.  A  number  of  these  engravings  have  been 
furnishe'd  by  BENJAMIN  PIKE,  jr.,  of  294  Broadway,  New  York,  and 
are  not  mere  fancy-sketches,  but  actual  representations  of  instru- 
ments (the  best  and  most  modern  of  their  kind)  manufactured  at 
his  establishment.  Mr.  Pike's  life  has  been  devoted  to  this  branch 
of  industry ;  and  it  may  not  be  improper  to  add  that  institutions 
desirous  of  procuring  a  set  of  apparatus,  partial  or  complete,  will 
find  his  assortment  unsurpassed  in  variety  and  excellence. — For 
convenience  of  recitation,  those  cuts  to  which  reference  is  made  by 
letters  are  reproduced  apart  from  the  text,  in  the  back  of  the  book. 

An  Alphabetical  Index  closes  the  volume 

NEW  YORK,  July  1,  1859. 


CONTENTS 


CHAPTEB 


PACK 


I. — MATTER  AND  ITS  FORMS         .        .        .       »>v      s«  *-  • 

II. — PROPERTIES  OP  MATTER 12 

III. — MECHANICS. 

Motion.— Momentum.— Striking  Force 26 

IV. — MECHANICS  (continued). 

Laws  of  Motion 34 

V.— MECHANICS  (continued). 

Gravity ....      46 

VI. — MECHANICS  (continued). 

Centre  of  Gravity ^ 0 

VII.— MECHANICS  (continued). 

The   Motive   Power.— The    Resistance.— The     Machine.— 

Strength  of  Materials .    "          81 

VIII. — MECHANICS  (continued). 

The  Mechanical  Powers         .  94 

IX.— MECHANICS  (continued). 

Wheel  work.— Clock  and  Watch  work 120 

X. — MECHANICS  (continued). 

Hydrostatics ^Q 

XI. — MECHANICS  (continued). 

Hydraulics      .        ...        .        .        .        .  .152 

XII.— PNEUMATICS , .,  .,  165 

XIII.— PYRONOMICS 192 

The  Steam  Engine !  .219 


(J  CONTENTS. 

CIIAPTEB  PAGE 

XIV— OPTICS         .        . •        •        •    229 

XV.— ACOUSTICS i     .•>•'-.•       •       .274 

XVI.— ELECTRICITY        ....  .      ....':   •      •  •*•-•        *        •        '  289 

Frictional  Electricity  .        .        .   jjfc    >  29° 

Voltaic  Electricity 316 

Thermo-electricity S32 

XVIL— MAGNETISM  .       ,.        .        .        ,      U,  ;  >       •        •        •        -    833 
Electro-magnetism        ...  ....    349 

Magneto-electricity S66 


XVIII.— ASTRONOMY 
XIX. — METEOROLOGY 


368 


FIGURES  REPRODUCED •       •       .407 


INDEX 


440 


LIBRARY 

UNIVERSITY  OF 

CALIFORNIA, 


NATURAL  PHILOSOPHY. 


CHAPTER  I. 

MATTER   ANI>    ITS    FORMS. 

1.  Matter. — Whatever  occupies  space,  whatever  we  can 
see  or  touch,  is  known  as  Matter.     Earth,  water,  air,  are 
different  forms  of  matter. 

A  distinct  portion  of  matter  is  called  a  Body.  The 
Earth,  a  ball,  a  rain-drop,  are  Bodies. 

2.  All  matter,  properly  speaking,  is  Ponderable, — that 
is,  has  weight. 

Imponderable  means  without  weight.  The  term  Imponderable  Matter  has 
been  applied  by  some  to  heat,  light,  electricity,  and  magnetism.  As  late  re- 
searches seem  to  indicate  that  these  are  forces  or  conditions  of  matter,  and  not 
themselves  varieties  of  matter,  they  are  now  generally  called  Imponderable 
Agents. 

3.  Forms  of  Ponderable  Matter. — Ponderable  Matter 
exists  in  three  forms  ;  Solid,  Liquid,  and  A-er'-i-form. 

A  body  is  said  to  be  Solid  when  its  particles  cohere,  so 
that  they  can  not  move  among  themselves ;  example,  ice. 
Solid  bodies  are  called  Solids. 


1.  What  is  Matter  ?  Give  examples.  What  is  a  Body  ?  Give  examples.  2.  What 
is  said  of  all  matter?  What  does  imponderable  mean?  To  what  has  the  term  im- 
ponderable matter  been  applied?  What  are  heat,  light,  electricity,  and  mag- 
netism generally  called  ?  8.  In  how  many  forms  does  ponderable  matter  exist  ? 


8  MATTER  AND  ITS  FORMS. 

A  body  is  said  to  be  Liquid  when  its  particles  cohere 
so  slightly  that  they  can  move  freely  among  themselves  ; 
example,  water.  Liquid  bodies  are  called  Liquids. 

Aeriform  means  having  the  form  of  air,  and  matter  is 
said  to  exist  in  this  state  when  its  particles  repel  each  other, 
tending  to  separate  and  spread  out  indefinitely  ;  example, 
steam.  Aeriform  bodies  are  called  Gases  and  Vapors. 

Liquid  and  aeriform  bodies  are  embraced  under  the 
general  name  of  Fluids. 

There  are  marked  points  of  difference  between  solids  and  fluids.  A  solid 
has  a  permanent  shape ;  a  fluid  accommodates  its  shape  to  that  which  con- 
tains it.  A  solid  may  often  be  moved  by  moving  a  portion  of  its  particles ; 
as  a  pitcher  by  its  handle.  The  particles  of  a  fluid,  on  the  other  hand,  do 
not  cohere,  and  therefore,  when  we  move  some  of  them,  the  rest  are  detached 
by  their  own  weight ;  thus  by  dipping  a  tumbler  into  a  pail  of  water,  we  can 
not  remove  all  the  fluid,  but  only  as  much  as  the  tumbler  contains.  Again,  a 
solid  resists  a  force  which  seeks  to  penetrate  it.  A  fluid,  on  the  contrary,  is 
easily  divided;  if  we  move  slowly  through  the  air,  for  instance,  we  feel  no 
resistance. 

The  same  substance  may,  under  different  circumstances,  appear  in  all 
three  of  these  forms.  Thus  water  is  a  liquid ;  when  frozen,  it  becomes  ice, 
which  is  a  solid ;  when  exposed  to  a  certain  degree  of  heat,  it  is  converted 
jfnto  steam,  which  is  aeriform. 

4.  Classes  of  Bodies. — Bodies  are  distinguished  as  Sim- 
ple and  Compound. 

A  Simple  Body  consists  of  matter  that  can  not  be  re- 
solved into  more  than  one  element ;  as,  gold. 

A  Compound  Body  consists  of  matter  that  can  be  re- 
solved into  two  or  more  elements ;  as  air,  which  is  com- 
posed of  two  gases. 

The  simple  bodies,  or  elements,  of  which  every  thing  in  the  universe  is 
composed,  are  sixty-two  in  number.  Of  these,  fifty,  distinguished  by  a  pe- 
culiar lustre,  are  called  Metals.  The  remaining  twelve  are  known  as  Non- 
metallic  Elements. 

Name  them.  When  is  a  body  said  to  be  solid  ?  What  are  solid  bodies  called  ?  When 
is  a  body  said  to  be  liquid  ?  What  are  liquid  bodies  called  ?  What  does  aeriform 
mean  ?  When  is  a  body  said  to  be  aeriform  ?  What  are  aeriform  bodies  called  ? 
What  name  is  applied  to  both  liquid  and  aeriform  bodies  ?  Mention  some  of  the 
marked  points  of  difference  between  solids  and  fluids.  In  how  many  forms  may  the 
same  substance  appear  ?  Give  an  example.  4  Into  how  many  classes  are  bodies  di- 
vided ?  Name  them.  What  is  a  Simple  body  ?  What  is  a  Compound  body  ?  How 
many  simple  bodies  are  there  ?  How  are  they  divided  ?  Name  the  principal  met* 


SIMPLE  SUBSTANCES.  9 

The  principqj  metals  are  the  seven  known  to  the  ancients, — gold,  silver, 
iron,  copper,  mercury,  lead,  and  tin ;  antimony,  which  was  next  discovered, 
in  1490 ;  bismuth,  zinc,  arsenic,  cobalt,  plat'-i-num,  nickel,  manganese,  &c. 
The  twelve  non-metallic  elements  are  ox'-y-gen,  hy'-dro-gen,  ni'-tro-gen, 
chlorine  [klo'-reen],  iodine  [i'-o-deeri],  bromine  [bro'-meeri],  fluorine  {Jlu'-o- 
reen],  se-le'-ni-um,  sulphur,  phosphorus,  carbon,  and  bo'-ron. 

These  simple  substances  are  rarely  found ;  nearly  every  body  that  we  meet 
with,  whether  natural  or  artificial,  is  composed  of  two  or  more  elements,  and 
is  therefore  compound.  Such  is  the  case  with  air,  which  was  anciently  thought 
to  be  a  simple  substance,  but  was  proved,  towards  the  close  of  the  eighteenth 
century,  to  be  a  mixture  of  21  parts  of  oxygen  and  79  parts  of  nitrogen. 
Water,  also,  has  been  found  to  be  a  compound  substance,  made  up  of  oxygen 
and  hydrogen  combined  in  the  proportion  of  1  to  8.  Of  the  sixty-two 
elements  referred  to  above,  twenty  are  so  rare  that  their  properties  are  not 
yet  fully  known;  thirty  more  are  comparatively  seldom  met  with;  the 
remainder  constitute  the  great  bulk  of  the  globe  and  all  that  is  thereon. 

The  consideration  of  the  simple  substances,  with  their 
properties  and  combinations,  belongs  to  the  science  of 
CHEMISTRY.  The  force  that  causes  them  to  combine  and 
produce  compound  substances,  is  called  Chemical  Affinity. 
Oxygen  and  hydrogen  combine  and  form  water,  in  conse- 
quence of  their  chemical  affinity. 

Chemical  affinity  subsists  only  between  certain  substances.  If  sulphuric 
acid  be  poured  on  a  piece  of  zinc,  the  two  substances  will  combine  and 
form  a  compound  entirely  different  from  either.  Pour  the  acid  on  a  lump  of 
gold,  and  no  such  change  will  ensue,  because  there  is  no  chemical  affinity  be- 
tween them. 

5.  Natural  Philosophy. — Natural  Philosophy  is  the 
science  that  treats  of  the  properties  and  laws  of  matter.  It 
is  also  called  PHYSICS. 

Pythagoras  was  the  first  to  use  the  term  philosophy.  From  him  and  his 
followers  it  was  borrowed  by  Socrates ;  who,  when  the  other  sages  of  his  time 
called  themselves  sophists,  or  wise  men,  modestly  declared  himself  a  philoso- 
pher, or  lover  of  wisdom, — Philosophy  implies  a  search  for  truth ;  and  Natu-  / 
ral  Philosophy,  as  distinguished  from  Moral  and  Intellectual  Philosophy, ' 
searches  for  the  truths  connected  with  the  material  world. 


als.  Name  the  twelve  non-metallic  elements.  What  is  said  of  the  simple  substances? 
What  kind  of  substances  are  air  and  water  ?  Of  what  is  air  composed  ?  Of  what, 
water  ?  How  many  elements  constitute  the  great  bulk  of  the  globe  ?  What  is  said 
of  the  rest  ?  To  what  science  does  the  consideration  of  the  simple  substances  be- 
long? What  causes  the  simple  substances  to  combine  ?  Give  an  instance  of  chem- 
ical affinity.  Illustrate  the  fact  that  chemical  affinity  subsists  only  between  certain 
substances.  5.  What  is  Natural  Philosophy  ?  With  whom  did  the  term  philosophy 
1* 


10  MATTER   AND   ITS   FOEMS. 

6.  Modes  of  Investigation. — We  arrive  at«the  facts  of 
Natural  Philosophy  in  two  ways ;  by  Observation  and  Ex- 
periment.    Observation  consists  in  watching  such  phenom- 
ena, or  appearances,   as   occur  in  the   course   of  nature. 
Experiment  consists  in  causing  such  phenomena  to  occur 
when  and  where  we  wish,  for  the  purpose  of  noting  the 
attendant  circumstances  and  results. 

For  example,  we  arrive  at  the  fact  that  an  unsupported  body  will  descend 
to  the  earth's  surface,  when  we  see  an  apple  fall  from  a  bough ;  this  is  by  Ob- 
servation. We  learn  the  same  fact,  when,  with  the  view  of  ascertaining 
what  it  will  do,  we  let  an  apple  drop  from  our  hands ;  this  is  by  Experiment. 

7.  Modes  of  Reasoning. — Having  obtained  our  facts  in 
the  two  ways  just  described,  and  classified  them,  we  next 
proceed  from  individual  cases  to  deduce  general  laws.   This 
is  called  Reasoning  by  Induction. 

Thus,  if  we  try  the  experiment  with  many  different  apples,  and  find  that 
each,  when  let  go,  will  fall  to  the  ground,  we  lay  down  the  general  law  that 
all  apples  will  fall  in  like  manner.  If  we  find  that  not  only  apples  do  this, 
but  also  other  objects  with  which  we  make  the  trial,  we  go  a  step  further, 
and  announce  another  law,  that  all  objects  left  unsupported  will  fall  to  the 
ground. 

It  is  by  this  process  that  most  of  the  laws  and  principles  of  Natural  Phi- 
losophy have  been  established.  Archimedes  [ar-ke-me'-deez],  the  Sicilian 
philosopher,  used  it  over  two  thousand  years  ago.  Gal-i-le'-o  revived  it  in 
modern  times,  and  it  may  be  said  to  lie  at  the  foundation  of  all  the  great  dis- 
coveries of  Newton. 

"When  we  have  two  similar  phenomena  and  know  that 
one  proceeds  from  a  certain  cause,  we  attribute  the  other 
to  the  same  cause.  This  is  called  Reasoning  by  Analogy. 

Such  reasoning  is  employed  in  the  case  of  all  bodies  that  are  beyond  our 
reach.  From  what  is  near,  we  draw  conclusions  respecting  what  is  remote. 
It  is  thus,  for  example,  that  the  philosopher  explains  the  motions  of  the  heav- 
enly bodies,  extending  to  them,  by  analogous  reasoning,  the  same  principles 
that  govern  the  motion  of  bodies  on  the  earth. 

8.  Division  of  the  Subject. — Natural  Philosophy,  hav- 


originate?  Who  borrowed  it  from  Pythagoras?  What  does  philosophy  imply? 
What  is  the  particular  province  of  Natural  Philosophy  ?  6.  How  do  we  arrive  at 
the  facts  of  Natural  Philosophy?  In  what  does  Observation  consist  ?  In  what,  Ex- 
periment ?  Illustrate  these  definitions.  7.  What  is  meant  by  reasoning  ~by  induc- 
tion f  Give  an  example.  By  what  philosophers  has  this  mode  of  reasoning  been 
employed  ?  What  ia  meant  by  reasoning  by  analogy  T  Give  an  example.  8,  What 


DIVISION   OP  THE   SUBJECT.  11 

ing  to  treat  of  matter  in  all  its  forms,  embraces  the  follow- 
ing distinct  sciences : — 

Mechanics,  which  treats  of  forces  and  their  application 
in  machines.  To  Mechanics  belong 

Hy-dro-stat'-ics,  which  treats  of  liquids  at  rest ; 

Hy-drau'-lics,  which  treats  of  liquids  in  motion. 

Pneumatics  \nu-mat' -ics\  which  treats  of  gases  and 
Vapors. 

Pyr-o-nom'-ics,  which  treats  of  heat. 

Optics,  which  treats  of  light  and  vision. 

Acoustics  \a-cow' -sties],  which  treats  of  sound. 

Electricity,  which  treats  of  the  electric  fluid.  To  Elec- 
tricity belong 

Galvanism,  which  treats  of  electricity  produced  by 
chemical  action ; 

Thermo-electricity,  which  treats  of  electricity  developed 
by  heat ; 

Magneto-electricity,  which  treats  of  electricity  devel- 
oped by  magnetism. 

Magnetism,  which  treats  of  magnets  and  the  forces  they 
develop.  To  Magnetism  belongs 

Electro-magnetism,  which  treats  of  magnetism  devel- 
oped by  electricity. 

Astronomy,  which  treats  of  the  heavenly  bodies. 

Me-te-o-rol'-o-gy,  which  treats  of  the  phenomena  of  the 
atmosphere. 


branches  does  Natural  Philosophy  embrace  ?  Of  what  does  Mechanics  treat  ?  Hy- 
drostatics? Hydraulics?  Pneumatics?  Pyronomics?  Optics?  Acoustics?  Elec- 
tricity? Galvanism?  Thermo-electricity?  Magneto-electricity?  Magmetisiu  ? 
Electro-magnetism ?  Astronomy?  Meteorology? 


12  PKOPEBTIES   OF  MATTER. 


CHAPTER  II. 

PROPERTIES    OF    MATTER." 

9.  EVERY  distinct  portion  of  matter  possesses  certain 
properties.     Some  of  these  belong  in  common  to  all  bodies, 
solid,  liquid,  and  aeriform,  and  are  called  Universal  Prop- 
erties of  matter.     Others,  again,  are  found  only  in  certain 
substances,  and  these  are  known  as  Accessory  Properties. 

The  Universal  Properties  of  matter  are  Extension,  Fig- 
ure, Impenetrability,  Indestructibility,  Inertia  [in-erf-sha], 
Divisibility,  Porosity,  Compressibility,  Expansibility,  Mo- 
bility, and  Gravitation. 

The  principal  Accessory  Properties  are  Cohesion,  Ad- 
hesion, Hardness,  Tenacity,  Elasticity,  Brittleness,  Mallea- 
bility, and  Ductilit^. 

We  proceecyto  consider  these  properties  in  turn. 

10.  EXTENSION. — Extension  is  that  property  by  which 
a  body  occupies  a  certain  portion  of  space.      The  portion 
of  space  thus  occupied  is  called  its  Place. 

In  other  words,  every  body,  however  small,  must  have  some  size,  or  a 
certain  length,  breadth,  and  thickness,  which  are  called  its  Dimensions. 
The  greatest  of  these  three  dimensions  is  its  Length ;  the  next  greatest,  its 
Breadth,  or  Width ;  the  least,  its  Thickness.  But,  instead  of  any  of  these 
terms,  we  use  the  word  TieigM  to  denote  distance  from  bottom  to  top  in  the 
case  of  objects  towering  above  us,  and  depth  to  denote  distance  from  top  to 
bottom  in  the  case  of  objects  extending  below  us. 

11.  FIGURE. — Figure  ia  that  property  by  which  a  body 
has  a  certain  shape. 

This  property  necessarily  follows  from  Extension  ;  for  since  every  body 
must  have  length,  breadth,  and  thickness,  it  must  also  have  some  definite 

9.  What  is  meant  by  Universal  Properties  of  matter  ?  What  is  meant  by  Acces- 
sory Properties  ?  Enumerate  the  universal  properties.  Mention  the  principal  ac- 
cessory properties.  10.  What  is  Extension  ?  What  is  meant  by  the  dimensions  of  a 
body  ?  What  is  Length  ?  Breadth  ?  Thickness  ?  When  are  the  terms  height  and 
depth  used  ?  11.  What  is  Figure  ?  From  what  does  figure  follow  ?  What  is  the 


13 


shape.  While  this  is  true  of  all  bodies,  it  must  be  remembered  that  the  form 
of  solids  is  permanent,  while  that  of  fluids  varies,  to  adapt  itself  to  every  new 
surface  with  which  it  comes  in  contact.  A  bullet  keeps  the  same  shape, 
wherever  it  is  placed ;  whereas  a  quantity  of  water,  poured  from  a  tumbler 
into  a  pail,  visibly  changes  its  form. 

12.  IMPENETRABILITY. — Impenetrability  is  that  property  / 
by  which  a  body  occupies  a  certain  portion  of  space,  to  the 
exclusion  for  the  time  of  all  other  bodies. 

Impenetrability  may  be  illustrated  with  a  variety  of  simple  experiments. 
Fill  a  tumbler  to  the  brim  with  water,  and  drop  in  a  bullet ;  the  water  will  at 
once  overflow.  Fill  a  bottle  with  water,  and  try  to  put  the  cork  in ;  the  cork, 
will  not  enter  till  it  has  displaced  some  of  the  water  :  if  it  fit  so  closely  that 
the  water  can  not  escape,  and  a  hard  pressure  be  exerted,  the  bottle  will 
burst. 

The  impenetrability  of  air  is  shown  with  the  ap- 
paratus represented  in  Figure  1.  A  is  a  glass  jar 
fitted  with  an  air-tight  cork,  through  which  a  funnel, 
B,  enters  the  jar.  C  is  a  bent  tube,  one  end  of  which 
also  passes  through  the  cork  into  the  jar,  while  the 
other  is  received  in  a  glass  of  water,  D.  Let  water 
be  poured  into  the  funnel ;  as  it  descends,  drop  by 
drop,  into  the  jar,  air  passes  out  through  the  benfc 
tube,  and  escapes  through  the  water  in  D  in  the  form* 
of  bubbles.  Thus  it  is  shown  that  water  and  air  can 
not  occupy  the  same  space  at  the  same  time. 

13.  Impenetrability  belongs  to  all  substances,  though  in  some  cases  it 
may  appear  to  be  wanting.  A  nail,  for  instance,  is  driven  into  a  piece  of 
wood  without  increasing  its  size ;  but  it  effects  an  entrance  by  forcing  to- 
gether the  fibres  of  the  wood,  not  by  occupying  their  space  at  the  same  time 
with  them.  In  like  manner,  a  certain  amount  of  salt  and  sugar  may  be  suc- 
cessively dropped  into  a  tumbler  brim-full  of  water  without  causing*  it  to  over- 
flow. The  particles  of  water,  which  are  supposed  to  be  globular,  do  not 
everywhere  touch  each  other,  and  the  particles  of  salt  are  accommodated  in 


the  interstices  between  them.  These  in  turn  leave  minute 
spaces,  into  which  the  still  smaller  particles  of  sugar  find  their 
way.  Fig.  2  exhibits  such  an  arrangement.  To  illustrate  it 
familiarly,  we  may  fill  a  vessel  with  as  many  oranges  as  it 
will  hold,  and  then  pour  on  a  quantity  of  peas,  shaking  the 
vessel  slightly  so  that  they  may  settle  in  the  empty  spaces. 


Fig.  2. 


difference  between  solids  and  fluids  as  regards  figure  ?  12.  "What  is  Impenetrability? 
Give  some  familiar  illustrations  of  this  property.  Describe  the  experiment  with  the 
apparatus  represented  in  Fig.  1.  13.  "What  is  said  of  those  cases  in  which  impen- 
etrability appears  to  be  wanting  ?  Illustrate  this  with  the  nail.  Explain  how  salt 
tnd  sugar  may  6e  dropped  into  a  tumbler  full  of  water  without  causing  it  to  over- 


14  PROPERTIES   OF   MATTER. 

When  the  vessel  will  receive  no  more  peas,  repeat  the  process  with  fine  grav- 
el, and  it  will  be  found  that  a  considerable  quantity  will  lodge  between  the 
oranges  and  peas. 

14.  INDESTRUCTIBILITY. — Indestructibility  is  that  prop- 
erty which  renders  a  body  incapable  of  being  destroyed. 

Matter  may  be  made  to  assume  a  new  form  and  new 
properties,  but  it  can  not  cease  to  exist.  The  quantity  of 
matter  now  in  the  world  is  precisely  the  same  as  when  it 
was  first  called  into  being,  and  it  will  continue  undimin- 
ished  till  the  end  of  time.  The  Deity  alone  created,  and  it 
is  only  He  that  can  destroy. 

15.  To  this  universal  law  we  have  some  apparent  exceptions;  but,  when 
closely  examined,  it  will  be  found  that  they  are  exceptions  in  appearance 
only.   Water,  for  instance,  exposed  to  the  air  in  a  shallow  dish,  will  at  length 
disappear  by  evaporation ;  but  it  is  not  destroyed.     Assuming  the  form  of 
vapor,  it  ascends,  becomes  incorporated  with  clouds,  is  condensed  into  rain, 
and  falls, — to  go  through  the  same  process  again. — The  oil  in  a  burning  lamp 
gradually  gets  lower  and  lower  till  at  last  it  is  all  gone,  and  we  say  it  is 
turned  up  ;  but  the  process  of  combustion,  or  burning,  only  changes  it  into 
invisible  gases, — not  one  particle  of  its  substance  is  lost.    In  like  manner, 
when  fuel  of  any  kind  is  consumed,  there  is  only  a  change  of  form,  not  a  de- 
struction of  the  least  portion  of  matter. 

Such  changes  are  constantly  going  on  in  the  operations  of  nature.  One 
body  perishes,  and  of  the  materials  that  composed  it  another  is  formed.  Our 
own  frames  may  contain  particles  that  were  in  the  bodies  of  Adam,  Noah,  or 
Socrates ;  or,  if  they  do  not  now,  may  do  so  to-morrow,  for  they  are  constant- 
ly parting  with  portions  of  their  substance,  the  place  of  which  is  as  con- 
stantly supplied  by  new  matter.  It  is  supposed  that  the  whole  body,  in- 
cluding even  the  innermost  parts  of  its  hardest  bones,  is  completely  renewed 
every  sever?  years.  Yet,  amid  all  the  countless  transitions  of  nature,  not  a 
single  particle  of  matter  is  destroyed  or  lost. 

16.  It  was  by  a  knowledge  of  the  indestructibility  of  matter  that  Sir  Wal- 
ter Raleigh-  is  said  to  have  won  a  wager  of  Queen  Elizabeth.   Having  weighed 
out  a  sufficient  quantity  of  tobacco  to  fill  his  pipe,  he  came  into  the  queen's 
presence,  aad  as  the  wreaths  of  smoke  curled  up  offered  to  bet  her  Majesty 
that  he  could  tell  their  weight.    Elizabeth  accepted  the  bet,  and  Sir  Walter 
quietly  finished  his  pipe ;  then,  having  shaken  out  the  ashes,  he  weighed 
them,  and,  subtracting  the  amount  from  that  of  the  tobacco  originally  put 


flow.  14.  What  is  Indestructibility  ?  What  can  be  done  to  matter,  and  what  not  ? 
15.  What  is  said  of  the  apparent  exceptions  to  this  law?  What  becomes  of  water 
exposed  to  the  air  ?  What  becomes  of  the  oil  in  a  burning  lamp  ?  What  is  said  of 
(ho  changes  of  nature  ?  What  is  said  of  the  changes  in  the  human  body  ?  16.  How 
did  Sir  Walter  Ealeigh  teach  Queen  Elizabeth  that  matter  is  indestructible  ?  17.  What 


INERTIA.  15 

iu,  told  the  queen  the  exact  weight  of  the  smoke.    Elizabeth  paid  the  wager, 
and  thus  learned  to  her  cost  that  matter  is  indestructible. 

1 7.  INERTIA.— Inertia  is  that  property  which  renders  a 
body  incapable  of  putting  itself  in  motion  when  at  rest,  or 
coming  to  rest  when  in  motion. 

When  a  stationary  body  begins  to  move,  or  a  moving 
body  comes  to  rest,  it  is  not  through  any  power  of  its  own, 
but  because  it  is  acted  on  by  some  external  agency,  which 
we  call  a  Force. 

That  no  inanimate  body  can  put  itself  in  motion,  is  evident  from  our  daily 
experience.  The  rocks  that  we  saw  on  the  earth's  surface  ten  years  ago  are 
to-day  in  precisely  the  same  place  as  they  then  were,  and  there  they  will  re- 
main forever  unless  some  force  removes  them. 

It  is  equally  true,  though  not  so  obvious,  that  a  body  once  in  motion  can 
not  of  itself  cease  to  move.  The  earth  revolves  on  its  axis,  the  heavenly 
bodies  move  in  their  orbits,  just  as  they  did  at  the  time  of  the  Creation;  they 
have  no  power  to  stop.  It  is  true  that  on  the  surface  of  the  earth  a  moving 
body  gradually  comes  to  rest,  when  the  force  which  put  it  in  motion  ceases 
to  act ;  but  this  is  owing  to  the  resistance  of  the  air  and  a  force  which  draws 
it  towards  the  centre  of  the  earth — not  to  any  agency  of  its  own.  Remove 
all  external  forces,  and  its  inertia  would  keep  it  moving  on  in  a  straight  line 
forever. 

18.  Familiar  Examples.— It  is  in  consequence  of  inertia  that  a  horse  has  to 
strain  hard  at  first  to  move  a  load,  which,  when  it  is  once  in  motion,  he  can 
draw  with  ease.    A  car,  through  its  inertia,  continues  moving  after  the  loco- 
motive is  detached.     Through  inertia,  a  person  standing  erect  in  a  stationary 
boat  or  wagon  is  thrown  backward  if  it  suddenly  starts  :  his  feet,  touching 
the  bottom,  are  carried  forward  with  it,  while  his  body  by  its  inertia  does  not 
partake  of  the,onward  motion  and  falls  backward.    So,  a  person  standing 
erect  in  a  boat  or  wagon  that  is  moving  rap-  _v 

idly,  is  thrown  forward  if  it  suddenly  stops ; 
his  feet  cease  to  move  at  once,  while  his  body 
continues  in  motion  in  consequence  of  its  iner- 
tia, and  falls  forward. 

19.  An  interesting  experiment  to  illustrate 
inertia  may  be  performed  with  the  apparatus 
represented  in  Fig.  3.    On  the  top  of  a  short 
pillar  is  placed  a  card,  and  on  the  card  a  brass 
ball.    Beside  the  pillar  is  fixed  a  steel  spring, 
with  an  apparatus  for  drawing  it  back.    If  the 

Is  Inertia?  "What  is  a  Force  ?  What  evidences  of  the  inertia  of  matter  have  we  in 
nature  ?  If  inertia  is  one  of  the  properties  of  matter,  why  does  a  moving  body  come 
Co  rest  on  the  earth's  surface  ?  18.  Giv«  some  familiar  examples  of  inertia  and  its 
Aonsequences.  19.  Describe  the  experiment  with  the  inertia  apparatus.  Describe 


16 


PROPERTIES    OF   MATTER. 


spring  is  drawn  back  and  then  suddenly  released,  it  will  drive  the  card  from 
the  top  of  the  pillar,  while  the  ball  in  consequence  of  its  inertia  will  retain 
its  place. 

Those  who  have  not  the  above  apparatus  may  balance  a  card  with  a  penny 
placed  upon  it  on  the  tip  of  one  of  the  fingers  of  the  left  hand,  and  strike  it 
pi     j  suddenly  with  the  middle  finger 

of  the  right  hand,  as  represented 
in  Fig.  4.  If  properly  balanced 
and  evenly  struck,  the  card  will 
fly  away,  and  the  penny  will  be 
left  on  the  finger. 

In  these  cases,  there  is  not 
sufficient  time  for  the  card  to 
overcome  the  inertia  of  the  ball 
and  the  penny,  and  impart  to 
them  its  own  motion.  When, 
however,  motion  has  once  been  communicated  by  one  body  to  another  rest- 
ing on  it,  the  inertia  of  the  latter  keeps  it  in  motion.  A  person  riding  in  a 
carriage  partakes  of  its  motion,  and  if  he  jumps  from  it  runs  the  risk  of  being 
thrown  down,  because  his  feet  cease  to  move  the  instant  they  strike  the 
ground,  while  the  inertia  of  his  body  carries  it  forward.  The  circus-rider 

takes  advantage  of  this  fact. 
While  his  horse 'is  going  at 
full  speed,  he  jumps  over  a 
rope  extended  across  the 
ring  (see  Fig.  5),  and  re- 
gains his  footing  on  the 
saddle  without  difficulty. 
To  do  this,  he  has  only  to 
leap  straight  up  as  he  comes 
to  the  rope,  for  his  inertia 
bears  him  along  in  the  same 
direction  as  his  horse. 
A  bullet  thrown  at  a  pane  of  glass  breaks  it  into  many  pieces,  but,  fired 
at  it  from  a  rifle,  merely  makes  a  circular  hole.  In  the  latter  case,  all  the  par- 
ticles of  glass,  on  account  of  their  inertia,  can  not  immediately  acquire  the 
rapid  motion  of  the  bullet;  and  consequently  only  that  portion  which  is 
struck  is  carried  onward.  On  the  same  principle,  a  thin  stick  resting  on  two 
wine-glasses  (see  Fig.  6)  may  be  broken  by  a  quick  blow  with  a  poker  in  its 
centre,  without  injury  to  its  brittle  supports. 


Fig.  5. 


the  experiment  with  the  card  and  penny.  What  is  the  effect  of  inertia,  when  motion 
has  onco  been  communicated  to  a  body?  Why  is  a  person  who  jumps  from  a  car- 
riage in  motion  thrown  down  ?  Explain  the  leap  of  the  circus-rider.  What  is  tho 
effect  of  throwing  a  bullet  against  a  pane  of  glass,  and  what  of  firing  it  ?  What  causes 
the  difference  ?  What  experiment  may  be  performed  to  illustrate  this  point  ?  20.  To 


DIVISIBILITY. 


17 


Fig.  7. 


20.  The  heavier  a  body  is, 
•the  greater  is  its  inertia  ;  the 
more  strongly  does  it  resist 
forces  that   would  set  it  in 
motion,  change  its  motion,  or 
stop  its  motion. 

Instinct  teaches  this  fact.  A  child, 
when  nearly  overtaken  by  a  man,  will 
suddenly  turn,  or  "  dodge"  as  he  calls 
it,  thus  gaining  ground,  inasmuch  as 
the  greater  weight  and  inertia  of  the  man  compel  him  to  make  a  longer  turn. 
So  a  hare,  in  making  for  a  cover,  often  escapes  a  hound  by  making  a  num- 
ber of  quick  turns.  The  greater  inertia  of  the 
hound  carries  him  too  far,  and  thus  obliges 
him  to  pass  over  a  greater  space,  as  seen  in 
Fig.  7,  in  which  the  continuous  line  shows  the 
hare's  path  and  the  dotted  line  the  hound's. 

2 1 .  DIVISIBILITY.  —  Divisibility 
is  that  property  which  renders   a 
body  capable  of  being  divided. 

Atomic  Theory.  —  Practically,  ^ 
there  is  no  limit  to  the  divisibility 
of  matter.  Most  philosophers,  how-  ^ 
ever,  hold  what  is  called  the  Atom- 
ic Theory, — that  if  we  had  more 
acute  senses  and  instruments  sufficiently  delicate,  we  would 
at  last,  in  dividing  and  subdividing  matter,  arrive  at  ex- 
ceedingly small  particles,  incapable  of  further  division. 
Such  particles  they  call  ATOMS,  a  term  derived  from  a 
Greek  word  meaning  indivisible. 

According  to  this  theory,  different  kinds  of  matter  are 
made  up  of  different  kinds  of  atoms ;  but  in  the  same  sub- 
stance the  atoms  are  always  the  same  in  shape  and  nature. 
It  must  be  remembered,  however,  that  no  particle  has  yet 
been  arrived  at  that  can  not  be  divided. 

22.  Instances  of  Divisibility. — Matter  has  been  divided  into  parts  incredi- 


what  is  a  body's  inertia  proportioned  ?  How  do  children  turn  this  fact  to  account? 
How  does  the  hare  apply  this  principle  ?  21.  What  is  Divisibility  ?  Is  there  any 
limit  to  the  divisibility  of  matter?  Give  the  chief  points  of  the  Atomic  Theory. 


18  PROPERTIES    OP  MATTER. 

bly  minute.  With  the  proper  instrument,  ten  thousand  distinct  parallel  lines 
can  be  drawn  on  a  smooth  surface  an  inch  in  width.  So  minute  are  these 
lines  that  they  can  not  be  seen  without  a  microscope,  not  even  a  scratch  be- 
ing visible  to  the  naked  eye. 

A  grain  of  musk  will  diffuse  a  perceptible  odor  through  an  apartment  for 
twenty  years.  It  does  this  by  filling  the  air  with  particles  of  its  substance  ; 
but  so  inconceivably  minute  are  these  particles,  that,  if  the  musk  is  weighed 
at  the  end  of  the  twenty  years,  no  loss  of  weight  can  be  detected. 

A  grain  of  copper  dissolved  in  nitric  acid  will  impart  a  blue  color  to  three 
pints  of  water.  Each  separable  particle  of  water  must  contain  a  portion  of 
the  grain  of  copper, — which  is  thus,  it  has  been  computed,  divided  into  no 
less  than  100,000,000  parts. 

23.  Nature  affords  many  striking  examples  of  the  divisibility  of  matter. 
The  spider's  web  is  so  attenuated  that  a  sufficient  quantity  of  it  to  go  around 
the  earth  would  weigh  only  eight  ounces ;  and  yet  this  minute  thread  con- 
sists of  about  a  thousand  separate  filaments. 

Blood  is  composed  of  small  red  globules  floating  in  a  colorless  liquid.  Of 
these  globules,  every  drop  of  human  blood  contains  at  least  a  million.  Mi- 
nute as  they  are,  they  may  be  divided  into  globules  much  more  minute.  As 
we  descend  in  the  scale  of  creation,  we  come  to  animals  whose  whole  bodies 
are  no  larger  than  these  little  globules  of  human  blood,  yet  possess  all  the 
organs  necessary  to  life.  How  inconceivably  small  are  the  vessels  through 
which  the  fluids  of  their  bodies  must  circulate ! 

The  microscope  reveals  to  us  wonders  of  animal  life  that  are  almost  in- 
credible. It  shows  us  in  duck-weed  animalcules  so  small  that  it  would  take 
ten  thousand  millions  of  them  to  equal  the  size  of  a  hemp-seed.  In  a  single 
drop  of  ditch-water,  it  exhibits  myriads  of  moving  creatures.  The  mineral 
called  tripoli  is  formed  of  these  animalcules  fossilized  or  turned  into  stone  ; 
and  it  has  been  shown  that  the  fortieth  part  of  a  cubic  inch  of  this  mineral 
contains  the  bodies  of  no  less  than  a  thousand  million  animalcules — or  more 
than  all  the  human  beings  on  the  globe. 

24.  POROSITY. — What  shape  the  atoms  of  different 
bodies  are,  we  have  no  means  of  determining.  By  reason 
of  their  shape,  however,  or  from  some  other  cause,  they  do 
not  everywhere  touch  each  other,  but  are  separated  by  in- 
terstices, to  which  we  give  the  name  of  Pores.  Pores  are 
often  visible  to  the  naked  eye,  as  in  sponge  and  pumice- 
stone  ;  in  other  cases,  as  in  gold  and  granite,  they  are  too 
Tninute  to  be  detected  even  with  the  microscope. 

Z2.  How  has  the  divisibility  of  matter  been  illustrated  with  a  smooth  surface  an  inch 
In  width  ?  How  does  a  grain  of  musk  prove  divisibility  ?  How,  a  grain  of  copper  ? 
23.  What  is  said  of  the  spider's  web?  Mention  some  examples  of  the  divisibility  of 
matter  afforded  by  nature.  What  does  the  microscope  reveal  to  us  ?  Mention  some 
of  these  wonders.  24.  What  are  Pores?  What  is  said  of  the  difference  in  the  size 


,  POROSITY.  19 

25.  Porosity  is  the  property  of  having  pores.  It  be- 
longs to  all  bodies. 

26.  That  water  is  porous,  is  proved  by  the  fact  that  a  vessel  filled  with  it 
will  receive  considerable  quantities  of  salt  and  sugar  without  overflowing. 
What  can  become  of  these  substances,  unless,  as  shown  in  Fig.  2,  their  par- 
ticles lodge  in  the  interstices  between  the  particles  of  water?    It  is  on  this 
principle  that  hot  water  receives  more  salt  and  sugar  without  overflowing 
than  cold.    Heat  expands  water, — that  is,  forces  its  particles  further  apart,— 
and  thus  enables  a  greater  quantity  of  salt  and  sugar  to  lodge  between  them. 

That  granite  is  porous,  is  shown  by  placing  a  piece  of  it  in  a  vessel  o{ 
water  under  the  receiver  of  an  air-pump  (described  on  page  178),  and  remov- 
ing the  air.  Little  bubbles  will  soon  be  seen  rising  through  the  water.  These 
bubbles  are  the  air  contained  in  the  invisible  pores  of  the  granite. 

A  piece  of  iron  is  made  smaller  by  hammering.  This  proves  its  porosity. 
Its  particles  could  not  be  brought  into  closer  contact,  if  there  were  no  inter- 
stices between  them. 

27.  An  experiment  performed  some  years  ago  at  Florence,  Italy,  to  ascer- 
tain whether  water  could  be  compressed,  proved  that  gold  is  porous.    A  vio- 
lent pressure  was  brought  to  bear  on  a  hollow  sphere  of  gold  filled  with  water. 
The  water  made  its  way  through  the  gold  and  appeared  on  the  outside  of  the 
sphere.    Water  will  thus  pass  through  pores  not  more  than  one  half  of  the 
millionth  of  an  inch  in  diameter. 

28.  Density  and  Rarity. — The  fewer  and  smaller  the 
pores  in  a  body,  the  more  compact  are  its  particles,  and 
the  greater  is  the  weight  of  a  given  bulk.     Bodies  whose 
particles  are  close  together  are  called  Dense;  those  with 
large  or  numerous  pores  are  called  Rare. 

29.  COMPRESSIBILITY  AND  EXPANSIBILITY. — These  two 
properties  are  the  opposites  of  each  other.    Compressibility 
is  that  property  which  renders  a  body  capable  of  being  re- 
duced in  size.    Expansibility  is  that  property  which  renders 
a  body  capable  of  being  increased  in  size. 

Compressibility  and  Expansibility  follow  from  porosity.. 
Since  the  particles  of  bodies  do  not  everywhere  touch  each 
other,  the  application  of  a  sufficient  force  will  bring  them 
closer  together,  and  the  size  of  the  bodies  will  thus  be  re- 

of  the  pores  ?  25.  What  is  Porosity  ?  26.  How  is  water  proved  to  be  porous  ?  Why 
does  hot  water  receive  more  salt  and  sugar  than  cold  ?  How  may  it  be  proved  that 
granite  is  porous?  How  is  the  porosity  of  iron  proved ?  27.  Give  an  account  of  the 
experiment  by  which  the  porosity  of  gold  was  proved.  How  small  pores  will  water 
pass  through  ?  23.  What  bodies  are  called  dense  t  What  bodies  are  called  rare  f 
29.  What  is  Compressibility  ?  What  is  Expansibility  ?  Show  how  these  properties 


20  PROPERTIES   OP  MATTER. 

duced.  A  sponge,  for  instance,  by  the  simple  pressure  of  the 
hand,  can  be  reduced  to  one-tenth  of  its  natural  size.  In  like 
manner,  if  the  pores  of  a  body  are  made  larger  by  any  agency 
(as  they  are  by  heat),  its  size  is  proportionately  increased. 

g  30.  All  bodies  possess  these  properties.  A  rod  of  iron, 

too  large  to  enter  a  certain  opening,  may  be  so  compressed  by 
hammering  as  to  pass  through  it,  and  then  so  expanded  by 
heat  as  to  render  its  entrance  again  impossible.  Liquids, 
which  were  long  considered  incompressible,  are  now  known 
to  yield  to  a  high  degree  of  pressure ;  their  expansibility  is 
illustrated  by  the  rise  of  mercury  in  the  thermometer. 

The  compressibility  and  expansibility  of  air  are  shown 
by  the  apparatus  represented  in  Fig.  8.  Let  P  be  a  piston, 
fitted,  air-tight,  to  the  cylinder  A  B.  As  the  piston  is  driven 
down,  the  air,  unable  to  escape,  is  compressed;  as  it  is 
drawn  back,  the  air  expands. 

Aeriform  bodies  are  more  easily  compressed  and  ex- 
panded than  any  others. 

31.  MOBILITY. — Mobility  is  that  property  which  renders 
a  body  capable  of  being  moved. 

Though  the  inertia  of  bodies  prevents  them'  from  mov- 
ing themselves,  yet  there  is  no  body  that  can  not  be  moved 
by  the  application  of  a  proper  force. 

32.  GRAVITATION. — Gravitation    (or   Gravity,  as   it  is 
called  when   acting  at   short  distances)   is  the   tendency 

Fig.  9.  which  one  body  has  to  approach  another,  under 
the  influence  of  the  latter's  attraction.  A  can- 
non ball  dropped  from  the  hand  falls  to  the  earth 
by  reason  of  its  gravity.  The  earth  at  the  same 
time  moves  towards  the  cannon  ball,  but  through 
a  space  inconceivably  small  in  consequence  of  its 
vast  superiority  in  size  over  the  ball. 

That  the  cannon  ball  is  capable  of  attracting  as  well  as  be- 
ing attracted,  may  be  proved  by  suspending  two  balls  close  to 
each  other  by  very  long  cords.  In  consequence  of  the  attrac- 
tion of  the  balls,  the  cords  will  not  hang  parallel,  but  will 
incline  towards  each  other  as  they  descend,  as  shown  in 


follow  from  porosity.  30.  How  may  compressibility  and  expansibility  bo  illustrated 
With  an  iron  rod  ?  What  is  said  of  these  properties  in  liquids  ?  How  may  the  com- 
pressibility and  expansibility  of  air  be  shown  ?  What  bodies  are  most  easily  com- 


COHESION. 


21 


Fig.  10. 


We  now  proceed  to  the  Accessory  Properties,  which 
are  confined  to  certain  bodies. 

33.  COHESION. — Cohesion  is  that  property  by  which  the 
particles  of  a  body  cling  to  each  other.    As  particles  are 
also  called  mol '-e-cules.  Cohesion  has  received  from  some 
authors  the  name  of  Mo-lec'-u-lar  Attraction. 

Cohesion  belongs  particularly  to  solids,  and  is  in  fact  the  cause  of  their 
solidity.  In  some  it  is  stronger  than  in  others,  rendering  them  harder  or 
more  tenacious.  Liquids  haye  so  little  cohesion  that  their  weight  alone  over- 
comes it,  and  causes  a  separation  of  particles.  In  aeriform  fluids  cohesion 
is  entirely  wanting,  its  place  being  supplied  by  a  Repulsive  Force,  which 
tends  to  make  their  particles  spread  out  from  each  other. 

34.  ADHESION. — Adhesion  is  that  property  by  which  the 
surfaces  of  two  different  bodies  placed  in  contact  cling  to- 
gether. 

The  bodies  in  question  may 
be  of  the  same  kind  of  mat- 
ter. This  is  proved  by  an  ex- 
periment with  two  glass  plates 
ground  perfectly  even.  Let 
these  be  pressed  together,  and 
it  will  be  found,  on  attempt- 
ing to  pull  them  apart  by  their 
handles,  that  considerable 
force  will  be  required.  The 
larger  the  surfaces  of  the 
plates,  the  harder  it  will  be  to 
separate  them.  A  pair  of  Ad- 
hesion Plates  is  represented 
in  Fig.  10. 

Adhesion  also  operates 
between  the  surfaces  of  sol- 
ids and  liquids.  Suspend  a 
piece  of  copper-plate  from 
one  side  of  a  pair  of  scales, 
in  such  a  way  that  its  under 
surface  may  be  parallel  to 
the  floor,  and  balance  it  with 
weights  placed  in  the  scale 
on  the  other  side.  Then, 
without  disturbing  the  cop- 


ADIIE8ION  PLATES. 


Fig.  11. 


pressed  and  expanded?  81.  What  is  Mobility?  82.  What  is  Gravitation?  How 
docs  it  operate  in  the  case  of  a  cannon  ball  dropped  from  the  hand  to  the  earth  ? 
How  does  it  operate  in  the  case  of  two  cannon-balls  suspended  close  to  each  other  ? 


22  PROPERTIES   OF   MATTER. 

per,  place  a  vessel  beneath  it,  as  in  Fig.  11,  and  pour  in  water  till  the  liquid 
just  reaches  the  plate.  The  adhesion  between  the  solid  and  the  liquid  is  now 
so  strong  that  additional  weights  (more  or  less,  according  to  the  extent  of 
surface)  may  be  put  in  the  scale  on  the  other  side  without  causing  them  to 
separate. 

35.  HARDNESS. — Hardness  is  that  property  by  which  a 
body  resists  any  foreign  substance  that  attempts  to  force  a 
passage  between  its  particles. 

The  hardness  of  a  body  depends  on  the  degree  of  firm- 
ness with  which  its  particles  cohere.  It  is  therefore  en- 
tirely distinct  from  density,  which  depends  on  the  number 
of  particles  in  a  given  bulk.  Thus  lead  is  dense,  but  not 
hard. 

Neither  liquids  nor  aeriform  fluids  possess  this  property;  and  even  in 
some  solids,  for  instance  butter  and  wax,  it  is  almost  entirely  wanting. 

Of  two  bodies,  that  is  the  harder  which  will  scratch  the  surface  of  the 
other.  By  trying  the  experiment  with  different  substances,  it  is  found  that 
the  precious  stones  are  harder  than  any  other  class  of  bodies,  the  diamond 
standing  first,  and  the  ruby,  sapphire,  topaz,  and  emerald  following  in  order. 
Rhodium  and  iridium  are  among  the  hardest  metals,  on  which  account  they 
are  used  for  the  tips  of  gold  pens. 

36.  TENACITY. — Tenacity  is  that  property  by  which  a 
body  resists  a  force  that  tends  to  pull  it  into  pieces. 

Both  hardness  and  tenacity  are  the  result  of  cohesion ; 
but  they  must  not  be  confounded.  Of  several  rods  equally 
thick,  that  which  will  support  the  greatest  weight  without 
breaking  is  the  most  tenacious  ;  that  which  it  is  most  diffi- 
cult to  cut  into,  is  the  hardest. 

The  metals  generally  are  remarkable  for  their  tenacity.  Some,  however, 
possess  this  property  in  a  higher  degree  than  others.  This  may  be  shown 
by  comparing  the  weights  which  different  metallic  wires  of  the  same  size 
are  capable  of  supporting.  An  iron  wire  one-tenth  of  an  inch  in  diameter 
will  sustain  nearly  550  pounds  without  breaking,  while  one  of  lead  will  be 
broken  by  a  weight  of  28  pounds. 

83.  What  is  Cohesion?  What  other  name  has  been  given  to  cohesion?  What 
is  said  of  cohesion  in  solids?  In  liquids?  In  aeriform  fluids?  34.  What  is 
Adhesion?  Describe  the  experiment  with  adhesion  plates.  Describe  the  exper- 
iment which  proves  that  adhesion  operates  between  solids  and  liquids.  85.  What 
is  Hardness?  "What  is  the  difference  between  hardness  and  density?  In  what 
is  hardness  wanting?  How  may  it  be  determined  which  of  two  bodies  is  the 
harder?  What  bodies  are  the  hardest  as  a  class?  Mention  the  order  in  which 
they  rank.  TV  hat  two  metals  are  distinguished  for  their  "hardness?  3C.  What 
is  Tenacity  ?  Of  what  are  both  hardness  and  tenacity  the  result  ?  Show  the  diffex- 


TENACITY. 


23 


Iron  is  the  most  tenacious  of  the  metals.  A  cable  of  this  material,  com- 
posed of  wires  one-thirtieth  of  an  inch  across,  will  support  the  enormous 
weight  of  60  tons  for  each  square  inch  in  its  transverse  section.  In  conse- 
quence of  this  great  tenacity,  such  cables  are  used  for  the  support  of  suspen- 
sion bridges. 

37.  Tenacity  of  Different  Substances. — It  is  important 
in  building  and  other  arts  to  know  the  relative  tenacity  of 
different  woods  and  metals.  To  determine  this,  experi- 
ments have  been  made.  Their  results  do  not  precisely 
agree,  inasmuch  as  there  are  differences  in  different  trees 
of  the  same  kind  and  different  pieces  of  the  same  metal ; 
yet  we  may  take  the  following  as  the  average  weights  that 
can  be  supported  by  the  several  materials  mentioned, — 
taking  in  each  case  a  rod  of  given  length  with  a  transverse 
section  of  a  square  inch. 


POUNDS. 

Metals.— Cast  Steel,  134,250 

Swedish  Iron,  72,000 

English  Iron,  55,800 

Cast  Iron,  19,000 

Cast  Copper,  19,000 

Cast  Tin,  .  4,700 

Cast  Lead,  1,825 


Woods.— Ash, 

Teak, 

Oak, 

Fir, 

Maple, 
Rope,  one  inch  around, 


POUNDS. 

14,000 

13,000 

12,000 

11,000 

8,000 

1,000 


Itope,  three  inches  around,  5,600 


It  is  a  curious  fact  that  a  composition  of  two  metals  may  be  more  tenacious 
than  either  of  them  separately.  Thus  brass,  which  is  made  of  zinc  and  cop- 
per, has  more  tenacity  than  either  of  those  metals. 

38.  The  liquids  have  comparatively  little  tenacity,  yet  there  is  a  differ- 
ence in  them  in  this  respect.  Milk,  for  instance,  is  more  tenacious  than  wa- 
ter ;  this  makes  it  boil  over  more  readily,  inasmuch  as  its  bubbles  do  not 
break,  but  accumulate,  climbing  one  upon  another  till  they  overtop  the  ves- 
sel. In  like  manner,  it  is  on  account  of  their  superior  tenacity  that  soap-suds 
will  make  a  lather  while  pure  water  will  not. 

39.  BKITTLENESS. — Brittleness  is  that  property  which 
renders  a  body  capable  of  being  easily  broken. 


ence  between  them.  What  is  said  of  the  tenacity  of  the  metals  ?  How  may  their 
relative  tenacity  be  shown  ?  Compare  iron  and  lead  in  this  respect.  What  is  said  of 
the  tenacity  of  iron  ?  87.  Explain  the  fact  that  experiments  for  determining  the  te- 
nacity of  different  substances  show  different  results.  What  does  the  table  show  ?  Of 
the  metals  mentioned  in  the  table,  which  has  the  greatest  tenacity  ?  Which,  the 
least  ?  Of  the  woods  mentioned,  which  is  the  most  tenacious  ?  Which,  the  least  ? 
What  curious  fact  is  mentioned  respecting  a  composition  of  two  metals?  38.  What 
Is  said  of  the  tenacity  of  liquids  ?  How  do  milk  and  water  compare  in  tenacity  ? 


24  PROPERTIES   OF  MATTER. 

Brittleness  is  the  opposite  of  tenacity,  but  often  charac- 
terizes hard  bodies.  Glass,  which  is  so  hard  that  it  will 
scratch  the  surface  of  polished  steel,  is  remarkable  for  its 
brittleness. 

A  substance  naturally  tenacious  may  be  so  treated  as  to  become  brittle. 
Thus  a  bar  of  iron  raised  to  a  high  degree  of  heat,  if  allowed  to  cool  gradu- 
ally, retains  its  tenacity,  and  bends  rather  than  breaks ;  but,  if  suddenly  cooled 
by  being  plunged  into  cold  water,  it  is  made  brittle. 

40.  ELASTICITY. — Elasticity  is  that  property  by  which  a 
body,  compressed,  dilated,  or  bent  by  an  external  force, 
resumes  its  form  when  that  force  has  ceased  to  act. 

Stretch  a  piece  of  india  rubber ;  when  you  let  go  the 
ends,  they  will  fly  back.  Bend  a  bow ;  when  the  string  is 
released,  the  bow  will  at  once  return  to  its  former  curve. 
These  are  familiar  examples  of  elasticity. 

41.  The  force  with  which  a  body  resumes  its  form  is  called  the  Force  of 
Restitution.    Those  bodies  whose  force  of  restitution  brings  them  back,  un- 
der all  circumstances,  exactly  to  their  original  form,  are  said  to  be  perfectly 
elastic.     The  only  perfectly  elastic  substances  are  the  aeriform  bodies.     A 
body  of  air  may  be  kept  compressed  for  years  ;  yet,  on  being  freed  from  the 
compressing  force,  it  will  immediately  expand  to  its  former  dimensions. 

42.  Many  of  the  hard  and  dense  solids  are  highly  elastic ;  for  example, 
steel,  marble,  and  ivory.   The  soft  solids  generally,  such  as  butter,  putty,  &c., 
have  little  or  no  elasticity ;  there  are  a  few,  however,  that  exhibit  it,  among 
which  are  india  rubber  and  silk  thread. 

43.  The  elasticity  of  steel  is  increased  by  making  it  suddenly  contract 
when  expanded  by  heat.     This  is  called  tempering,  and  is  effected  by  raising 
the  steel  to  an  intense  heat,  plunging  it  in  cold  water,  and  keeping  it  there 
for  a  certain  time.     The  process  is  a  nice  one.    At  Damascus,  in  Syria,  and 
Toledo,  in  Spain,  it  was  long  performed  with  peculiar  skill,  so  that  the  sword- 
blades  of  those  two  cities  were  considered  superior  to  all  others.    At  the 
World's  Fair  in  London,  a  Toledo  sword  was  exhibited,  of  such  exquisite 
temper  that  it  could  be  bent  into  a  circle,  yet  on  being  released  sprung  back 
and  became  as  straight  as  ever. 

44.  A  compound  of  two  metals  may  possess  a  higher  degree  of  elasticity 

Soap-suds  and  water  ?  39.  "What  is  Brittleness  ?  Of  what  is  brittleness  the  opposite ? 
What  is  said  of  glass?  How  may  iron  be  made  brittle?  4Q.  What  is  Elasticity? 
Give  some  familiar  examples.  41.  What  is  meant  by  the  Force  of  Eestitution? 
When  is  a  body  said  to  be  perfectly  elastic  ?  What  are  the  only  perfectly  elastic  sub- 
•tances  ?  42.  "What  solids  are  for  the  most  part  elastic,  and  what  not  ?  43.  How  is 
the  elasticity  of  steel  increased  ?  What  is  this  process  called  ?  Describe  the  process 
of  tempering.  Where  was  it  long  done  with  peculiar  skill  ?  Give  an  account  of  the 
Toledo  blade  exhibited  at  the  World's  fail'.  44.  What  is  said  of  a  compound  of  two 


ELASTICITY. 


25 


than  either  of  them  separately.    Thus  bell-metal  is  much  more  elastic  than 
either  the  tin  or  the  copper  of  which  it  is  composed. 

45.  An  elastic  body,  thrown  against  any  hard  substance, 
rebounds.  An  india  rubber  ball  bounds  back  from  a  wall, 
to  a  distance  proportioned  to  the  force  with  which  it  is 
thrown.  In  such  cases,  the  ball  is  flattened  at  the  point  of 
contact,  but  instantly  resumes  its  former  shape  with  such 
force  as  to  drive  the  ball  back. 

To  prove  this,  take  two  ivory  balls  (Fig.  12),  smear  one  of  Fig.  12. 
them  with  printer's  ink,  and  suspend  them  near  each  other  by 
strings  of  equal  length.  Bring  them  gently  in  contact,  and  a 
few  particles  of  ink  will  adhere  to  the  surface  of  the  clean 
ball :  strike  them  violently  together,  and  a  larger  spot  of  ink 
will  be  found  there.  This  could  not  happen  if  the  two  balls 
were  not  flattened  at  the  moment  of  striking. 

46.  There  is  a  limit  to  the  elasticity  of  most  bodies,  beyond 
which,  if  compressed,  dilated,  or  bent,  they  will  fail  to  regain 
their  original  form.  An  iron  wire,  if  slightly  bent,  springs 
back,  so  that  no  change  of  form  can  be  detected  ;  but  not  so, 
if  violently  bent.  A  continued  application  of  the  compressing, 
dilating,  or  bending  force,  has  the  same  effect.  A  bow,  if  kept 
bent  for  a  long  time,  will  lose  its  elasticity.  For  this  reason, 
an  archer,  before  putting  his  bow  away,  is  careful  to  un- 
string it. 

47.  The  liquids  have   but   little   elasticity.     They  are 
therefore  called  Non-elastic  Fluids ;  while  aeriform  bodies, 
which  possess  this  property  in  a  higher  degree  than  any 
others,  are  known  as  Elastic  Fluids. 

48.  MALLEABILITY. — Malleability  is  tha>  property  which 
renders  a  body  capable  of  being  rolled  out  or  hammered 
into  sheets. 

From  a  piece  of  copper,  a  workman  with  no  other  instrument  than  his 
hammer  will  make  a  hollow  vessel  without  joint  or  seam,  the  malleability  of 
the  metal  preventing  it  from  giving  way  under  his  blows.  Dough,  which 
can  be  made  into  very  thin  sheets  under  the  rolling-pin,  affords  a  familiar 
illustration  of  malleability. 

Malleability  belongs  chiefly  to  the  metals,  yet  in  some  of  them,  such  as 
antimony  and  bismuth,  it  is  wanting.  It  is  strikingly  exhibited  in  silver, 


o 


metals  ?  Give  an'  example.  45.  "What  does  an  elastic  body  do,  when  thrown  against 
a  hard  substance  ?  In  such  cases,  what  takes  place  ?  Prove  this  by  an  experiment. 
46.  What  is  said  of  the  limit  of  elasticity  ?  Give  examples.  47.  What  names  have 
been  given  to  liquids  and  auriform  bodies  ?  Why  ?  48.  What  is  Malleability  ?  Give 

'     2 


26  MECHANICS. 

platinum,  iron,  and  copper,  but  most  of  all  in  gold.  A  cubic  inch  of  this  met- 
al may  be  beaten  out  till  it  covers  282,000  square  inches,  which  makes  the  leaf 
only  o-^V TTO"  °f  an  m°k  tmck'  In  other  words,  it  would  take  282,000  strips 
of  such  gold  leaf,  lying  on  each  other,  to  make  the  thickness  of  an  inch. 

49.  DUCTILITY. — Ductility  is  that  property  which  ren- 
ders a  body  capable  of  being  drawn  out  into  wire. 

The  malleable  metals  are  for  the  most  part  ductile,  but 
not  always  in  the  same  degree.  Thus  gold  exceeds  all  the 
other  metals  in  ductility  as  well  as  in  malleability ;  but  tin, 
which  can  readily  be  beaten  into  very  thin  sheets,  can  not 
be  drawn  out  into  small  wire. 

Gold  wire  has  been  made  so  attenuated  that  fifty  miles  of  it  would  weigh 
but  an  ounce.  Platinum,  which  is  nearly  as  ductile  as  gold,  has  been  drawn 
into  wire  only  -5$^-$-$  of  an  inch  in  diameter  and  invisible  to  the  naked  eye. 
Glass,  when  softened  by  fire,  becomes  exceedingly  ductile,  and  may  be  spun 
out  into  flexible  and  elastic  threads  scarcely  larger  than  the  thread  of  the 
silk- worm. 


.CHAPTER  III. 

0  >>  MEG  HANICS. 

_         .Nics  is  that  branch  of  Natural  Philosophy 
which  treat!  t)f  forces  and  their  application  in  machines. 

51.  FoEClfAND  RESISTANCE. — When  we  see  a  body  be- 
gin to  move,  cease  to  move,  or  change  its  motion,  since  it 
can  do  neither  of  itself,  we  know  that  it  has  been  acted  on 
by  some  external  agency,  which  we  call  a  Force.  The 
elasticity  of  a  bow  which  sends  an  arrow  through  the  air, 
is  a  force  ;  the  wind,  which  changes  its  direction,  is  a  force ; 
gravity,  which  brings  it  to  the  earth  and  helps  to  stop  its 
motion,  is  a  force. 

examples.  To  what  does  malleability  chiefly  belong  ?  Sho  *r  the  extreme  malleabil- 
ity of  gold.  49.  What  is  Ductility  ?  What  substances  are  for  thfe-afiost  part  ductile  ? 
What  is  the  most  ductile  substance  known  ?  What  facts  are  state/d,  illustrating  the 
ductility  of  gold,  platinum,  and  glass? 

50.  What  is  Mechanics?    51.  What  is  a  Force  ?    Give  illustrations.    What  is  th* 


MOTION.  27 

That  which  opposes  a  force  is  called  the  Resistance. 
In  the  above  example,  the  inertia  of  the  arrow  is  the  re- 
sistance. 

Forces  may  act  on  bodies  so  as  to  produce  either  Mo- 
tion or  Rest. 

Motion. 

52.  Motion  is  a  change  of  place. 

53.  Motion  is  either  Absolute  or  Relative. 

*  Absolute  Motion  is  a  change  of  place  with  reference  to 
a  fixed  point.  Relative  Motion  is  a  change  of  place  with 
reference  to  a  point  that  is  itself  moving. 

Two  balls  are  rolled  on  the  floor.  The  motion  of  each,  as  regards  the  point 
from  which  it  was  thrown,  is  absolute ;  their  motion  with  reference  to  each 
other  is  relative. 

54.  REST. — Rest  is  the  opposite  of  motion,  and  implies 
continuance  in  tl^  same  place. 

Like  motion,  Rest  is  either  Absolute  or  Relative.  A 
man  sitting  on  a  steamer  tfyat  is  moving  forward  five  feet 
in  a  second,  is  at  rest  relatively  to  the  other  objects  on 
board.  To  be  at  rest  absolutely,  he  must  walk  five  feet 
every  second  towards  the  stern  of  the  boat. 

Strictly  speaking,  there  is  no  such  thing  as  absolute  rest  in  any  of  the  ob- 
jects that  surround  us ;  for  the  earth  moves  round  the  sun  at  the  rate  of 
nearly  99,000  feet  in  a  second,  and  carries  with  it  every  thing  on  its  surface. 
Hills,  trees,  and  houses,  therefore,  though  they  occupy  the  same  place  with 
respect  to  each  other,  are  really  travelling  through  space  with  immense  ra- 
pidity. Yet  as  this  is  the  case  with  ourselves,  with  the  atmosphere,  and  all 
things  about  us,  we  regard  an  object  as  absolutely  at  rest  if  it  has  no  other 
motion  than  this. 

55.  VELOCITY. — The  .Velocity  of  a  body  is  the  rate  at 
which  it  moves. 

This  rate  is  determined  by  the  space  it  passes  over  in  a 
given  time.  The  .greater  the  space,  the  greater  the  velo- 
city. Thus,  if  A  walks  two  miles  an  hour,  and  B  four,  B's 
velocity  is  twice  as  great  as  A's. 

Resistance  ?  What  may  the  action  of  forces  on  bodies  produce  ?  52.  What  is  Motion  ? 
53.  How  is  motion  distinguished  ?  What  is  Absolute  Motion  ?  What  is  Relative 
Motion  ?  Illustrate  these  definitions.  54.  What  is  Best  ?  Illustrate  Absolute  and 
Relative  Eest  Show  that  there  is  really  no  such  thing  as  absolute  rest,  55.  What  is 


28  MECHANICS. 

56.  The  relation  between  the  space  passed  over>  the 
time  employed,  and  the  velocity,  is  such,  that  when  two 
are  given,  we  can  find  the  third. 

Rule  1. — To  find  the  velocity  of  a  body,  divide  the 
space  passed  over  by  the  time. 

Example.    A  locomotive  goes  120  miles  in  4  hours  ;  what  is  its  velocity  ? 
— Dividing  120  by  4,  we  get  30 ;  answer,  30  miles  an  hour. 

Rule  2. — To  find  the  time,  divide  the  space  by  the  ve- 
locity. 

Example.    A  locomotive  goes  120  miles  at  the  rate  of  30  miles  an  hour; 
how  long  is  it  on  the  way  ? — Dividing  120  by  30,  we  get  4 ;  answer,  4  hours. 

Rule  3. — To  find  the  space,  multiply  the  velocity  by 
the  time. 

ExampJe..    A  locomotive  goes  4  hours  at  the  rate  of  30  miles  an  hour ;  how 
far  does  it  travel  ? — Multiplying  30  by  4,  we  get  120 ;  answer,  120  miles. 

57.  Table  of  Velocities. — It  may  not  be  uninteresting 
to  compare  the  average  velocities  of  the  following  moving 
objects : — 

Miles  per  hour. 

A  hurricane 80 

Sound 764 

A  musket-ball,  when  first 

discharged 850 

A  rifle-ball 1,000 

A  24-lb.  cannon-ball 1,600 

Earth  in  its  orbit , . .  68,040 

Light 691,200,000 

Electric  Fluid 1,036,800,000 


Miles  per  hour. 

A  man  walking 3 

A  horse  trotting 7 

A  slow  river 3 

A  rapid  river 7 

A  fast  sailing  vessel 10 

A  fast  steamboat 18 

A  railroad  train 25 

A  moderate  wind *. .  7 

A  storm 50 


58.  KINDS  OF  MOTION. — There  are  three  kinds  of  mo- 
tion ;  Uniform,  Accelerated,  and  Retarded. 

59.  Uniform  Motion  is  that  of  a  body  which  moves  over 
equal  spaces  in  equal  times. 

Uniform  motion  would  be    produced  by  a   force  acting  once  and  then 

Velocity?  How  is  it  determined?  56.  What  is  said  of  the  relation  between 
the  space,  the  time,  and  the  velocity  ?  Give  the  rule  for  the  velocity,  and 
example.  Give  the  rule  for  the  time,  and  example.  Give  the  rule  for  the  space, 
and  example.  57.  What  is  the  velocity  of  a  slow  river?  A  rapid  river?  A  mod- 
erate wind  ?  A  hurricane  ?  Sound  ?  Light  ?  The  electric  fluid  ?  A  rifle-ball  ?  Tho 
earth  in  its  orbit?  58.  Name  the  three  kinds  of  motion.  59.  What  is  Uniform  Mo- 


KINDS    OF   MOTION.  29 

ceasing  to  act,  if  the  moving  body  were  free  from  all  other  influences,  for 
its  inertia  would  keep  it  moving  at  the  same  rate.  Gravity  and  the  re- 
sistance of  the  air,  however,  constantly  retard  a  moving  body ;  and,  there- 
fore, to  keep  up  a  uniform  motion,  a  force  just  sufficient  to  nullify  these  re- 
tarding agencies  must  continue  acting.  There  are  very  few  cases  of  uniform 
motion  either  in  nature  or  art. 

60.  Accelerated  Motion  is  that  of  a  body  whose  velo* 
city  keeps  increasing  as  it  moves.     It  is  produced  by  the 
continued  action  of  a  force. 

A  ball  dropped  from  a  height  is  a  familiar  instance  of  accelerated  motion. 
The  moment  it  is  let  go,  the  attraction  of  gravitation  causes  it  to  descend. 
Were  this  force  and  every  other  then  suspended,  the  ball  would  fall  to  the 
earth  with  a  uniform  motion  ;  but  gravity,  continuing  to  act,  forces  it  along 
faster  and  faster,  and  thus  imparts  to  it  an  accelerated  motion. 

A  body  is  said  to  have  a  Uniformly  Accelerated  Mo- 
tion, when  its  velocity  keeps  increasing  at  the  same  rate ; 
when,  for  instance,  it  moves  two  feet  in  the  first  second, 
four  in  the  next,  eight  in  the  third,  &c. 

61.  Retarded  Motion  is  that  of  a  body  whose  velocity 
keeps  diminishing  as  it  moves.     It  is  produced  by  the  con- 
tinued action  of  some  resistance  on  a  moving  body. 

A  ball  rolled  over  the  ground,  under  the  continued  action  of  gravity  and 
the  resistance  of  the  air,  moves  more  and  more  slowly,  till  finally  it  comes  to 
rest.  This  is  an  example  of  retarded  motion. 

A  body  is  said  to  have  a  Uniformly  Retarded  Motion, 
when  its  velocity  keeps  diminishing  at  the  same  rate ; 
when,  for  instance,  it  moves  eight  feet  in  the  first  second, 
four  in  the  next,  and  two  in  the  third. 

Momentum. 

62.  The  Momentum  (plural,  momenta)  of  a  body  is  its 
quantity  of  motion. 

A  ten-pound  ball,  moving  at  the  rate  of  400  feet  in  a  second,  may  be  sup- 
posed to  be  divided  into  ten  pieces,  each  weighing  one  pound.  Each  piece 
has  a  motion  of  400  feet  in  a  second ;  and  the  quantity  of  motion,  or  momen- 

tion?  Theoretically,  how  is  uniform  motion  produced?  Practically,  how  is  it  pro- 
duced  ?  60.  What  is  Accelerated  Motion  ?  How  is  it  produced  ?  Give  an  example 
of  accelerated  motion.  When  is  a  body  said  to  have  a  Uniformly  Accelerated  Motion  r 
61.  What  is  Retarded  Motion  ?  How  is  it  produced  ?  Give  an  example.  When  is  a 
body  said  to  have  a  Uniformly  Eetarded  Motion  ?•  62.  What  is  Momentum  ?  Give 


30  MECHANICS. 

turn,  of  all  ten,  that  is,  of  the  whole  ball,  will  be  ten  times  400,  or  4,000. 
Hence  the  following  rule : — 

63.  Rule. — To  find  the  momentum  of  a  moving  body, 
multiply  its  velocity  by  its  weight. 

Example.  What  is  the  momentum  of  a  ten-pound  ball,  moving  at  the  rate 
of  400  feet  in  a  second  ?— Multiplying  400  by  10,  we  get  4,000;  answer,  4,000. 

64.  When  the  momenta  of  different  objects  are  to  be  compared,  their  weight 
and  velocity  must  be  expressed  in  units  of  the  same  denomination  :  if  the 
weight  of  one  is  given  in  pounds,  that  of  the  other  must  be  in  pounds;  if  the 
velocity  of  one  is  so  many  feet  per  second,  that  of  the  other  must  be  expressed 
in  feet  per  second.     If  different  denominations  are  given,  reduce  them  to  the 
same  denomination. 

Thus:  A  weighs  50  pounds,  and  has  a  velocity  of  7,200  miles  an  hour;  B 
weighs  100  pounds,  and  has  a  velocity  of  4  miles  a  second.  Which  has  the 
greater  momentum  ? 

3,600  seconds  make  an  hour ;  and  if  A's  velocity  is  7,200  miles  an  hour,  in 
a  second  it  will  be  ^g^o"  of  7,200  miles,  or  2  miles. 

A's  weight    50  multiplied  by  A's  velocity  2  gives  A's  momentum  100. 
B's  weight  100  multiplied  by  B's  velocity  4  gives  B's  momentum  400. 
Therefore  B's  momentum  is  4  times  as  great  as  A's. 

65.  Two  bodies  of  the  same  weight  have  momenta  proportioned  to  their 
velocities.     Thus,  if  two  balls  weighing  5  pounds  each,  move  respectively  at 
the  rate  of  20  and  10  miles  an  hour,  then  their  momenta  will  be  in  the  pro- 
portion of  20  to  10,  or  two  to  one. 

Two  bodies  moving  with  the  same  velocity,  have  momenta  proportioned 
to  their  weight.  Thus,  if  two  balls  moving  at  the  rate  of  5  miles  an  hour, 
weigh  20  and  10  pounds  respectively,  then  their  momenta  will  be  in  the  pro- 
portion of  20  to  10,  or  two  to  one. 

66.  Since  momentum  depends  on  velocity  as  well  as  weight,  it  is  obvious 
that,  by  increasing  its  velocity  sufficiently,  a  small  body  may  be  made  to 
have  a  greater  momentum  than  a  large  one.     Thus,  a  bullet  fired  from  a  gun 
has  a  greater  momentum  than  a  stone  many  times  larger  thrown  from  the 
hand. 

On  the  same  principle,  a  very  heavy  body,  though  its  motion  may  be 
hardly  perceptible,  may  have  an  immense  momentum.  This  is  the  case 
with  icebergs,  rendering  them  fatal  to  objects  with  which  they  come  in  col- 
lision. 

an  example.  63.  Kopeat  the  rule  for  finding  a  body's  momentum.  Give  an  example. 
64.  When  the  momenta  of  different  objects  are  to  be  compared,  what  is  essential? 
Give  an  example.  65.  "When  two  bodies  have  the  same  weight,  to  what  are  their 
momenta  proportioned  ?  Give  an  example.  "When  two  bodies  have  the  same  velo- 
city, to  what  are  their  momenta  proportioned?  Give  an  example.  66.  How  may  a 
greater  momentum  be  given  to  a  small  body  than  a  large  one  ?  Illustrate  this.  How 
CD  you  account  for  the  great  momenta  of  icebergs,  notwithstanding  their  slow  mo- 


STRIKING  FOECE.  31 


Striking  Force. 

67.  The  Striking  or  Living  Force  of  a  -moving  body  is 
the  force  with  which  it  strikes'  a  resisting  substance. 

Striking  Force  is  sometimes  confounded  with  momen- 
tum, but  improperly,  inasmuch  as  it  is  the  product  of  the 
weight  into  the  square  of  the  velocity.  Two  moving  bodies 
may  have  the  same  momentum,  but  differ  greatly  in  their 
striking  force. 

Thus,  the  ball  A,  weighing  200  pounds  and  moving  2  miles  a  minute,  has 
a  momentum  of  200  multiplied  by  2,  or  400.  The  ball  B,  weighing  20  pounds 
and  moving  20  miles  a  minute,  also  has  a  momentum  of  400  (20  multiplied 
by  20).  How  do  they  compare  in  striking  force  ?  That  of  A  is  equal  to  its 
weight  200  multiplied  by  the  square  of  its  velocity,  4,— or  800.  That  of  B  is 
equal  to  its  weight  20  multiplied  by  the  square  of  its  velocity  400, — or  8,000. 
Therefore,  though  the  momenta  of  the  two  balls  are  equal,  the  striking  force 
of  B  is  10  times  as  great  as  that  of  A ;  if  both  were  fired  into  a  bank  of  moist 
clay,  B  would  penetrate  ten  times  as  far  as  A. 

68.  As  the  velocity  of  a  body  increases,  its  striking 
force  increases  also,  but  in  a  higher  degree. 

If,  for  instance,  a  train  of  cars  be  moving  50  miles  an  hour,  and  another 
train  of  the  same  weight  10  miles  an  hour,  the  striking  force  of  the  former 
will  not  be  to  that  of  the  latter  as  50  to  10,  but  as  the  squar'  of  50  is  to  the 
square  of  10,  or  as  2500  is  to  100.  The  former  train  would  therefore  do  25 
times  as  much  damage  as  the  latter  to  any  object  with  which  it  came  in  col- 
lision, or  to  itself  in  case  of  being  thrown  from  the  track.  This  result  is  borne 
out  by  facts. 

69.  Rule. — To  find  the  striking  force  of  a  moving  body, 
multiply  its  weight  into  the  square  of  its  velocity. 

If  the  striking  force  of  one  body  is  to  be  compared 
with  that  of  another,  see  that  their  weight  and  velocity  are 
in  units  of  the  same  denomination. 

Example.    The  stone  A,  weighing  1  pound,  is  thrown  at  the  rate  of  20  ft. 


tion  ?  67.  WTiat  Is  meant  by  the  Striking  or  Living  Force  of  a  moving  body  ?  What 
is  the  difference  between  a  body's  striking  force  and  its  momentum  ?  Exemplify  this 
difference.  68.  How  does  a  body's  striking  force  increase,  compared  with  its  veloci- 
ty? Give  an  example.  How  is  this  result  borne  out  ?  69.  Give  the  rule  for  finding 
the  striking  force  of  a  moving  body.  When  bodies  are  to  be  compared  with  respect 
to  their  striking  force,  how  must  their  weight  and  velocity  be  expressed  ?  Solve  the 
axample  under  the  rule. 


32  MECHANICS. 

a  second.  The  stone  B,  weighing  3  pounds,  is  thrown  at  the  rate  of  2,400  ft. 
a  minute.  Which  will  penetrate  further  into  a  snow-bank  ? 

20  times  20  is  400  =  square  of  A's  velocity. 

400  X  1  (A's  weight)  =  400,  A's  striking  force. 

Reduce  B's  velocity  to  the  same  denomination  as  A's.  If  B  move  2,400 
feet  in  a  minute,  in  a  second  it  will  move  ^L  of  2,400  feet,  or  40  feet. 

40  times  40  is  1,600  =  square  of  B's  velocity. 

1,GOO  X  3  (B's  weight)  =  4,800,  B's  striking  force. 

Ans.— A's  striking  force  being  400,  and  B's  4,800,  B  will  penetrate  into 
the  snow-bank  12  times  as  far  as  A. 


EXAMPLES   FOK   PRACTICE. 

1.  (See  Rule  1,  §  56.)    A  fox-hound  will  run  30  miles  in  three  hours.    What 

is  its  velocity  ? 

2.  At  the  battle  of  Brandywine,  Gen.  Greene's  detachment  marched  4  miles 

in  42  minutes,  to  relieve  Gen.  Sullivan.  With  what  velocity  did  they 
move? 

3.  At  the  most  flourishing  period  of  its  history,  ancient  Athens  was  25  miles 

in  circumference.  With  what  velocity  would  an  Athenian  have  had  to 
move,  in  order  to  walk  round  the  city  in  5  hours  ? 

4.  A  pigeon  will  fly  100  miles  in  2  hours.     What  is  its  velocity  ? 

5.  P  walks  2  miles  in  30  minutes ;  Q  walks  4  miles  in  2  hours.    Which  has 

the  greater  velocity  ? 

REMARK. —  When  different  denominations  are  used,  they  must  be  reduced  to 
the  same  denomination,  as  shown  in  §  64. 

6.  The  current  of  a  rapid  river  runs  1,200  feet  in  2  minutes ;  ahorse  at  a  mod- 

erate trot  passes  over  30  feet  in  3  seconds.  Which  moves  with  the 
greater  velocity  ? 

7.  (See  Rule  2,  §  56.)    Strabo  tells  us  that  ancient  Nineveh  was  47  miles  in 

circumference  ;  in  what  time  could  a  person  have  walked  around  it,  at 
the  rate  of  10  miles  a  day  ? 

8.  The  bombardment  of  Ostend,  on  the  coast  of  Holland,  was  heard  in  Lon- 

don, a  distance  of  70  miles.  There  are  5,280  feet  in  a  mile,  and  sound 
travels  at  the  rate  of  1,120  feet  in  a  second.  How  many  seconds  after  a 
cannon  was  fired  at  Ostend,  was  the  report  heard  in  London  ? 

9.  From  the  base  of  the  Pyramid  of  Cheops  to  its  top  is  704  feet ;  how  long 

will  it  take  a  person  to  ascend  it,  walking  at  the  rate  of  4  feet  per  second  ? 

10.  A  rifle-ball  moves  at  the  rate  of  1,000  miles  an  hour.    If  it  could  maintain 
the  same  speed,  how  long  would  it  be  in  crossing  the  Atlantic  Ocean, 
which  is  3,000  miles  broad  ? 

11  Light  moves  192,000  miles  in  a  second,  electricity  288,000  miles  in  the 
same  time.  How  long  before  we  could  see  a  flash  of  lightning  in  a  cloud 
2  miles  off,  and  how  long  before  the  lightning  could  strike  an  object  by 
our  side  ? 

12.  In  the  year  1804,  the  French  philosopher  Gay  Lussac  ascended  in  a  bal- 


EXAMPLES   FOR  PRACTICE.  33 

loon  to  the  height  of  4y2  miles.    He  came  down  at  the  rate  of  660  feet  in 
a  minute ;  how  long  was  he  in  making  the  descent  ? 

13.  (See  Rule  3,  §  56.)    Some  of  the  Alpine  glaciers  move  25  feet  annually. 
How  far  would  they  move  in  4  years  ? 

14.  The  comet  observed  by  Newton  in  1680  moved  880,000  miles  an  hour. 
How  far  at  this  rate  would  it  move  in  a  day  ? 

15.  Which  will  pass  over  the  greater  space — a  hurricane,  moving  at  the  rate 
of  80  miles  an  hour  in  4  hours,  or  a  locomotive,  going  30  miles  an  hour, 
in  10  hours? 

16.  If  the  earth  moves  in  its  orbit  68,040  miles  an  hour,  and  is  365  days,  6 
hours,  in  completing  its  revolution,  how  long  is  its  orbit? 

17.  If  a  ray  of  light  travels  691,200,000  miles  in  an  hour,  how  far  will  it  go 
in  a  day  ? 

18.  (See  Rules,  §§  63,  69.)    A  24-pound  cannon-ball  moves  at  the  rate  of  1,000 
miles  an  hour.     A  battering-ram  weighing  10,000  pounds  moves  at  the 
rate  of  10  miles  an  hour.     How  do  their  momenta  compare  ? — Ans.  As 
24  to  100 ;  that  is,  the  cannon-ball  has  a  little  less  than  one-fourth  of  the 
momentum  of  the  battering-ram. 

How  does  the  striking  force  of  the  above  cannon-ball  compare  with  that 
of  the  battering-ram ;  that  is,  what  would  be  their  comparative  effect  on 
the  wall  of  a  fortress  ? — Ans.  Z7iat  of  the  ball  would  be  24  times  as  great 
as  that  of  the  battering-ram. 

19.  An  iceberg  weighing  50,000  tons  moves  at  the  rate  of  2  miles  an  hour. 
An  avalanche  of  10,000  tons  of  snow  descends  with  a  velocity  of  10  miles 
an  hour.     How  do  their  momenta  compare  ? 

How  do  they  compare  in  striking  force  ? 

20.  How  does  the  momentum  of  a  32-pound  ball  with  a  velocity  of  2,000  miles 
an  hour,  compare  with  that  of  a  16-pound  ball  with  a  velocity  of  1,000 
miles  an  hour? 

Which  would  penetrate  further  into  a  bank  of  moist  clay  ? 

21.  A  locomotive  weighing  20  tons  moves  with  a  velocity  of  40  feet  a  second. 
Another  locomotive  weighing  25  tons  moves  at  the  rate  of  4,800  feet  in  a 
minute.    How  do  their  velocities  compare  ? 

How  do  they  compare  in  momentum  ? 

If  the  one  with  the  less  striking  force  penetrate  10  feet  into  a  snow- 
bank, how  far  will  the  other  penetrate  ? 

E2.  A  stone  weighing  15  ounces  is  thrown  from  the  hand  with  a  velocity  of 
1,320  feet  in  a  minute.  A  rifle-ball  weighing  3  ounces  is  discharged  at 
the  rate  of  15  miles  a  minute.  How  do  their  velocities  compare  ? 

How  do  they  compare  in  momentum  ? 

How  many  times  greater  is  the  striking  force  of  the  rifle-ball  than  thak 
of  the  stone? 

2* 


MECHANICS. 


CHAPTER  IV. 


MECHANICS   (CONTINUED). 


LAWS   OF  MOTION. 

Mathematical  Definitions. 

70.  BEFOEE  treating  of  the  laws  of  motion,  it  is  neces- 
sary to  define  the  mathematical  terms  used  in  connection 
with  them. 

Fig.  13. 


Fig.  14. 


Fig.  15. 


1.  A  Right  or  Straight  Line  is  one  that  has  the  same 
direction  throughout  its  whole  extent ;  as,  AJJ. 

2.  Parallel  Lines  are  those  which  have  the  same  direc- 
tion ;  as,  C  D,  E  F. 

3.  A  Curve  Line,  or  Curve, -is  one  that  changes  its  di- 
rection at  every  point;  as,  GH. 

4.  A  Circle  is  a  figure  bounded  by  a  curve,  every  point 
of  which  is  equally  distant  from  a  point  within,  called  the 
Centre.    Fig.  16  represents  a  circle,  and  E  its  centre. 

5.  The  Circumference  of  a  circle  is  the  curve  that 
bounds  it ;  as,  A  C  F  B  D. 

6.  Any  part  of  the   circumference  is  called  an 
Arc ;  as,  A  C,  C  F. 

7.  A  Diameter  of  a  circle  is  a  straight  line  drawn 
through  the  centre,  terminating  at  both  ends  in  the 
circumference ;  as,  A  B.    Every  circle  has  an  infinite 
number  of  diameters,  all  equal  to  each  other. 

8.  A  Radius  (plural,  radii}  of  a  circle  is  a  straight  line  drawn  from  the 
centre  to  the  circumference ;  as,  ED,  E  C,  E  F,  E  A,  E  B.    Every  circle  has 
an  infinite  number  of  radii,  all  equal  to  each  other.    The  radius  of  a  circle  is 
just  half  its  diameter. 

9.  A  Tangent  of  a  circle  is  a  straight  line  that  touches  the  circumference 


70.  What  is  a  Eight  Line  ?  What  are  Parallel  Lines  ?  What  is  a  Curve  Line  ? 
What- is  a  Circle  ?  What  is  the  Circumference  of  a  circle  ?  What  is  an  Arc  ?  What 
is  a  Diameter  of  a  circle  ?  How  many  diameters  has  every  circle  ?  What  is  a  Eadius? 
How  many  radii  has  every  circle  ?  How  does  the  radius  of  a  circle  compare  with  its 


MATHEMATICAL   DEFINITIONS.  35 

In  a  single  point,  without  cutting  it  at  either  end  when  pro-  Fig.  It. 

duced ;  as,  A  B,  C  D. 

10.  The  circumference  of  every  circle  is  divided  into  360 
equal  parts,  called  Degrees.     One  fourth  of  the  circumfer- 
ence contains  90  degrees,  and  is  called  a  Quadrant. 

11.  An  Angle  is  the  difference  in  direction  of  two  straight 
lines  that  meet  or  cross  each  other. 

12.  The  Vertex  (plural,  vertices}  of  an  angle  is  the  point  at  which  its  sides 
meet ;  as,  D  in  Fig.  18. 

An  angle  is  named  from  the  letter  at  its  vertex,  if  but  Fig.  18. 
one  angle  is  formed  there.  Otherwise,  it  is  named  from 
the  letters  on  each  side  and  at  the  vertex,  that  at  the  vertex 
being  placed  in  the  middle.  Thus  the  angle  in  Fig.  18  is 
called  D;  if  more  than  one  angle  were  formed  there,  it 
would  be  distinguished  as  C  D  B  or  B  D  C. 

The  size  of  an  angle  does  not  depend  on  the  length  of  its  sides,  but  sim- 
ply on  their  difference  of  direction.  We  may  extend  the  lines  DC,  D  B,  as 
far  as  we  choose,  without  making  the  angle  D  any  larger. 

13.  When  a  straight  line  meets  another  straight  line  in  such  a  way  as  to 
make  the  two  adjacent  angles  equal,  that  is,  so  as  to  incline  no  more  to  one 
side  than  the  other,  it  is  said  to  be  Perpendicular 

to  the  latter ;  and  the  angle  which  it  makes  on  either 
side  is  called  a  Right  Angle.  Thus,  F  E  B  and  F  E  A 
(being  equal)  are  Right  Angles,  and  the  line  F  E  is 
Perpendicular  to  the  line  A  B. 

—  A  right  angle,  it  will  be  seen,  is  measured  by 
one  fourth  of  the  circumference  of  a  circle,  or  90  de- 
grees. 

14.  An  Obtuse  Angle  is  one  that  is  greater  than  a 
right  angle ;  as,  F  E  D  in  Fig.  19. 

15.  An  Acute  Angle  is  one  that  is  less  than  a  right  angle ; 
as,  F  E  C  in  Fig.  19. 

16.  A  Triangle  is  a  figure  bounded  by  three  straight 
lines  ;  as,  A  B  C,  Fig.  20. 

17.  A  Quadrilateral  is  a  figure  bounded  by  four  straight 
lines ;  as,  A  B  C  D,  Fig.  21. 

18.  A  Diagonal  of  a  quadrilateral  is  a  straight  line 
which  joins  the  vertices  of  two  opposite  angles ;  as,          l, 
A  C,  D  B,  in  Fig.  21.  / 

19.  A  Parallelogram  is  a  quadrilateral  whose  oppo-     /^_ 

site  sides  are  parallel ;  as,  A  B  C  D,  Fig.  21.  A  B 

diameter  ?  What  is  a  Tangent  of  a  circle  ?  How  is  the  circumference  of  every  circle 
divided  ?  What  is  a  Quadrant  ?  What  is  an  Angle  ?  What  is  the  Vertex  of  an  an- 
gle? How  is  an  angle  named?  On  what  alone  does  the  size  of  an  angle  depend? 
When  is  one  line  said  to  be  Perpendicular  to  another  ?  What  is  a  Eight  Angle  ?  By 
what  is  a  right  angle  measured  ?  What  is  an  Obtuse  Angle  ?  What  is  an  Acute  An- 


36  MECHANICS. 

20.  A  Rectangle  is  a  quadrilateral  whose  angles  are 

all  right  angles ;  as,  E  F  G  H,  Fig.  22.  Fi<y  2g 

21.  A  Square  is  a  rectangle  whose  sides  are  equal ;      i°    j 
as,  UK  L,  Fig.  23.  , 1 

22.  A  Sphere  is  a  solid  bounded  by  a  curved  surface, 

all  the  points  of  which  are  equally  distant  from  a  point  within  called  L  K 
the  centre ;  as,  A  B  C  D,  Fig.  24. 

23.  The  Axis  of  a  sphere  is  a  straight  line 
passing  through  its  centre  and  terminating  in  its 
surface,  round  which  it  revolves ;  as,  the  straight 
line  connecting  A  and  B,  in  Fig.  24. 

24.  The  Poles  of  a  sphere  are  the  extremities 
of  its  axis ;  as,  the  points  A,  B,  in  Fig.  24. 

25.  The  Equator  of  a  sphere  is  a  great  circle 
which  we  imagine  to  be  drawn  round  it  on  its 
surface,  midway  between  the  poles ;  as,  the  cir- 
cle C  D,  in  Fig.  24. 

26.  An  Oblate  Spheroid  is  a  figure  which  dif- 
fers from  a  sphere  only  in  being  flattened  at  its 
poles,  like  an  orange. 

27.  A  Prolate  Spheroid  is  a  figure  which  differs  from  a  sphere  only  in  be- 
ing lengthened  out  at  its  poles,  like  a  lemon. 

28.  A  Cylinder  is  a  circular  body  of  uniform  diameter,  the  ends  of  which 
form  equal  and  parallel  circles.    A  lead-pencil,  before  it  is  sharpened,  is  a 
cylinder ;  a  stove-pipe  is  a  hollow  cylinder. 

71.  By  investigating  the  principles  of  motion,  Newton 
arrived  at  three  great  laws,  which  have  ever  since  been 
received. 

First  t,aw  of  Motion. 

72.  A  body  at  rest  remains  at  rest,  a  body  in  motion 
moves  in  a  straight  line  with  uniform  velocity ',  unless  acted 
on  by  some  external  force. 

This  law  follows  from  inertia.  No  body  has  power  of  itself  to  move,  to 
cease  moving,  or  to  change  its  direction  or  velocity. 

73.  The  air  is  a  powerful  agent  in  stopping  motion.  This  is  shown  by 
causing  a  wheel  to  revolve  on  a  pivot,  first  in  the  air,  and  then  under  a  glass 

gle  ?  What  is  a  Triangle  ?  What  is  a  Quadrilateral  ?  What  is  a  Diagonal  of  a  quad- 
rilateral? What  is  a  Parallelogram?  What  is  a  Rectangle?  What  is  a  Square? 
What  is  a  Sphere  ?  What  is  the  Axis  of  a  sphere  ?  What  are  the  Poles  of  a  sphere  ? 
What  is  the  Equator  of  a  sphere  ?  What  is  an  Oblate  Spheroid  ?  What  is  a  Prolate 
Spheroid  ?  What  is  a  Cylinder  ?  71.  How  many  laws  of  motion  did  Newton  arrive 
at?  72.  What  is  the  First  Law  of  Motion?  From  what  does  this  law  follow? 
73.  How  may  it  be  shown  that  the  air  is  a  powerful  agent  in  stopping  motion  ? 


FIRST  LAW   OF   MOTION.  37 

receiver  from  which  the  air  has  been  removed  with  an  air-pump.  In  the  for- 
mer case,  the  wheel  soon  ceases  to  move ;  in  the  latter,  it  retains  its  motion 
for  a  long  time.  A  pendulum  (see  §  138)  will  vibrate  nearly  a  day  in  an  ex- 
hausted receiver. 

74.  Friction  is  the  resistance  with  which  a  body  meets  from  the  surface 
on  which  it  moves.  The  rougher  the  surfaces  brought  in  contact,  the  great- 
er the  friction,  and  the  sooner  the  moving  body  will  come  to  rest.  A  ball 
rolled  over  a  stony  road  is  soon  stopped  by  the  obstacles  it  encounters ;  on  a 
level  pavement  it  goes  much  farther,  and  farther  still  on  a  smooth  sheet  of 
ice.  This  is  because  the  friction  becomes  less  in  proportion  as  the  surface 
on  which  the  ball  rolls  becomes  smoother. 

75.  According  to  this  law,  every  body  left  free  to  obey 
the  force  that  set  it  in  motion  will  move  in  a  straight  line. 
We  observe  few  such  motions  in  nature.     The  planets  in 
their  orbits,  rivers  in  their  channels,  rolling  waves,  and  as- 
cending smoke,  ah1  move  in  curves,  in  consequence  of  their 
being  acted  on  by  other  forces,  besides  those  that  set  them 
in  motion.     The  tendency  of  the  moving  body,  however, 
is  always  to  continue  in  a  straight  line,  even  when  from 
overruling  causes  it  moves  in  a  circle. 

Attach  a  ball,  for  instance,  to  a  cord ;  and, 
fastening  the  end  of  the  cord  at  a  point,  0,  give 
a  quick  impulse  to  the  ball.  It  will  be  found  to 
move  in  a  circle,  A  B  C  D,  because  the  cord  keeps 
it  within  a  certain  distance  of  the  centre.  Were 
it  not  for  this,  it  would  move  in  a  straight  line. 
Thus,  let  the  cord  be  cut  when  the  ball  is  at  A, 
and  it  will  be  found  to  move  to  E  in  a  tangent  to 
the  circle  A  B  C  D.  In  like  manner,  at  B  it  will 
fly  off  in  a  tangent  to  F,  and  so  at  C,  D,  or  any 
other  point. 

76.  THE  CENTRIFUGAL  FORCE. — The  force  which  tenda 
to  make  a  body  fly  from  the  centre  round  which  it  revolves, 
is  called  the  Cen-trif-u-gal  Force. 

The  opposite  force,  which  draws  a  body  towards  the 
centre  round  which  it  revolves,  is  called  the  Cen-trip'-e-tal 
Force. 

Magnificent  examples  of  these  two  forces  are  exhibited 

74.  What  is  Friction  ?  On  what  kind  of  surfaces  does  a  moving  body  encounter  th« 
most  friction  ?  Exemplify  this.  75.  What  is  said  of  the  motions  that  we  find  in  na- 
ture ?  Give  some  instances.  What  is  the  tendency  of  the  moving  body  ?  Illustrate 
this  with  a  ball  and  cord.  76.  What  id  the  Centrifugal  Force  ?  What  is  the  Centrip. 


38 


MECHANICS. 


Fig.  26. 


by  the  planets  revolving  round  the  sun  in  space.  At  each 
successive  point  of  their  orbits,  in  obedience  to  the  Cen- 
trifugal Force,  they  tend  to  fly  off  in  tangents,  disturbing 
the  harmony  of  the  universe  and  carrying  desolation  in 
their  path.  They  are  constantly  restrained,  however,  by  a 
Centripetal  Force  equally  powerful,  the  sun's  attraction; 
and  the  result  is  that  they  revolve  in  curves. 

77.  Familiar  Examples. — Whirl  a  wet  mop  rapidly  round, 
and  drops  of  water,  propelled  by  this  force,  will  fly  off  from 
it  in  straight  lines. 

Suspend  a  glass  vessel  containing  some  colored  water,  by 
a  cord  passed  round  the  rim,  as  shown  in  Fig.  26.  Turn  the 
vessel  round  till  the  cords  become  tightly  twisted,  and  then 
suddenly  let  it  go.  It  will  rapidly  revolve,  and  the  centrifu- 
gal force  will  give  the  water  an  impulse  away  from  the  centre. 
As  it  can  not  escape,  it  will  spread  up  the  sides.  Should  there 
be  water  enough,  it  will  rise  above  the  top  of  the  vessel,  and 
fly  off  in  straight  lines. 

We  take  advantage  of  the  centrifugal  force  in  discharging 
a  stone  from  a  sling.     The  stone  is  whirled  quickly  round  the 
hand  as  a  centre,  which  it  is  prevented  from  leaving  by  two 
strings  connected  with  the  strap  on  which  it  rests.    The  in. 
stant  one  of  the  strings  is  let  go,  the  centrifugal  force  carries 
off  the  stone  in  a  tangent  to  the  circle  it  was  describing.    Its 
direction  varies  according  to  the  point  at  which  the  string  is  let  go,  as  will 
appear  from  Fig.  27.    Great  velocity  may  be  communicated  to  the  stone  with 
this  simple  apparatus.     In  the  hands  of  the  Per- 
sians, the  Rhodians,  and  other  ancient  nations, 
the  sling  was  a  formidable  weapon. 

When  a  wagon  turns  a  corner  rapidly,  it  is 
liable  to  be  upset  in  consequence  of  the  centrif- 
ugal force.  A  person  sitting  in  it  feels  his  body 
sway  outward,  and  one  who  is  on  his  feet  has 
to  grasp  the  wagon  to  avoid  being  thrown  from 
his  place.  To  counteract  the  effects  of  the  cen- 
trifugal force  in  curves  on  railroads,  the  outer 
rail  is  laid  higher  than  the  inner  one,  as  repre- 
sented in  Fig.  28.  Were  it  not  for  this  precau- 


Fig.  2T. 


«tal  Force  ?  What  examples  of  these  two  forces  does  nature  furnish  us  ?  77.  Ho\r 
may  a  mop  be  made  to  illustrate  the  centrifugal  force  ?  How  does  the  apparatus  rep- 
resented in  Fig.  26  illustrate  the  Centrifugal  Force  ?  Describe  the  mode  in  which  a 
etone  is  discharged  from  a  sling,  and  explain  the  principle.  What  Is  the  effect  of  tho 
centrifugal  force,  when  a  wagon  turns  a  corner  rapidly  ?  How  is  this  effect  counter- 


THE   CENTRIFUGAL   FORCE. 


39 


Fig.  29. 


tion,  trains  moving  swiftly  round  a  curve  Fig.  28. 

would  often  be  thrown  from  the  track. 

Instinct  teaches  a  horse  running  rapidly 
round  a  small  circle,  to  incline  his  body  in- 
ward, that  he  may  counteract  the  centrifugal 
force.  For  the  same  reason,  a  circus-rider, 
going  swiftly  round  the  ring,  has  to  lean  to- 
wards the  centre. 

Jugglers  take  advantage  of  the  centrifu- 
gal force  to  astonish  their  audiences  with  a 
striking  experiment,  represented  in  Fig.  29. 
A  B  is  a  wheel  with  a  broad  rim,  or  felly.    A 
wine-glass  partly  filled  with  water  is  placed 
on  the  inner  surface  of  the  felly,  and  the  wheel 
is  then  made  to  revolve  rapidly  round  the 
axle  0.    If  the  proper  amount  of  motion  be  communicated 
to  the  wheel,  not  only  will  the  wine-glass  keep  its  place 
on  the  felly,  but  the  water  also  will  remain  in  it,  not  a 
drop  being  spilled,  even  when  the  glass  is  at  W.     Grav- 
ity, which,  if  the  wheel  were  stationary,  would  at  once 
cause  both  glass  and  water  to  fall,  is  completely  nullified 
by  the  centrifugal  force. 

78.  Law  of  the  Centrifugal  Force. — The 
centrifugal  force   of   a  revolving  body  in- 
creases according  to  the  square  of  its  velocity.     If,  there- 
fore, the  earth  revolved  round  the  sun  twice  as  fast  as  it 
now  does,  its  centrifugal  force  would  be  4  times  as  great ; 
if  3  times  as  fast,  9  times  as  great;  if  4  times  as  fast,  16 
times  as  great,  &c. 

This  explains  why  a  cord  with  which  a  stone  is  whirled  round,  as  in  a 
eling,  is  more  apt  to  break  under  a  rapid  motion  than  a  slow  one.  Every 
time  the  velocity  is  doubled,  the  strain  on  the  cord  is  increased  fourfold. 

79.  Effect  of  the  Centrifugal  Force  on  Revolving  ^Bod- 
ies.— The  centrifugal  force  acts,  not  only  on  bodies  moving 
in  curves,  but  also  on  fixed  bodies  revolving  on  their  own 
axes. 

When  large  wheels  are  turned  rapidly  by  machinery, 
the  centrifugal  force  at  the  circumference  becomes  an  agent 


acted  in  railroads  ?  How  does  instinct  teach  a  horse  to  counteract  the  centrifugal 
force  ?  Describe  the  juggler's  trick  performed  with  the  aid  of  the  centrifugal  force. 
78.  What  is  the  law  of  the  centrifugal  force  ?  When  is  the  cord  of  a  sling  most  apt  to 
break,  and  why  ?  79.  On  what,  besides  bodies  moving  in  curves,  does  the  centrifugal 


40 


MECHANICS. 


Fig.  30. 


of  tremendous  power.  Unless  such  wheels'  are  made  of 
very  strong  materials,  their  cohesion  will  be  overcome  by 
the  centrifugal  force,  and  they  will  fly  into  fragments.  Pon- 
derous grindstones  sometimes  burst,  with  the  most  disas- 
trous effects,  when  too  great  a  velocity  is  imparted  to  them. 

Fig.  30  represents  a  sphere 
revolving  on  its  axis.  All  parts 
of  the  surface  have  to  complete 
their  revolution  in  exactly  the  same 
time ;  therefore,  as  the  parts  lying 
on  the  equator  CD  are  further  from 
the  axis,  and  have  a  greater  distance 
to  go,  they  must  travel  faster  than 
the  rest.  Now  we  have  seen  that 
the  centrifugal  force  increases  with 

the  square  of  the  velocity ;  and,  therefore,  at  the  equator 
C  D  it  will  be  stronger  than  at  any  other  part  of  the  sur- 
face. 

Hence  the  general  law: — On  a  revolving  sphere,  the 
centrifugal  force  is  greatest  at  the  equator,  and  diminishes 
from  that  point  till  at  the  poles  it  wholly  disappears. 

Fig.  31.  80.  This  difference  of  intensity  in  the  cen- 

trifugal force  at  different  points  is  shown  whon 
a  sphere  of  moist  clay  is  made  to  revolve  rapid- 
ly, as  on  a  potter's  wheel.  The  tendency  of  par- 
ticles on  and  near  the  equator  to  fly  off  is  so  great 
that  in  those  parts  the  sphere  bulges  out,  becom- 
ing-proportionately  flattened  at  the  poles. 

A  similar  result  is  produced  in  the  apparatus 
represented  in  Fig.  31.  Two  thin  and  flexible 
metal  hoops  are  fixed,  at  right  angles  to  each 
other,  on  the  axis  E  F, — fastened  at  the  end  F, 
but  loose  at  E,  so  as  to  admit  of  their  moving 
freely  up  and  down  the  rod  E  F.  A  rapid  rotary 
motion  being  communicated  to  the  hoops,  they  will  assume  an  oval  form, 
bulging  Out  more  and  more  as  their  velocity  is  increased.  When  allowed 
to  come  to  rest,  they  will  rise  to  their  original  position  at  E. 

force  act?  What  is  sometimes  its  effect  on  large  wheels  moved  by  machinery  ?  What 
is  the  law  of  the  centrifugal  force  in  the  case  of  revolving  spheres  ?  Explain  the  rea- 
Bon  of  this.  80.  What  is  the  effect  of  the  centrifugal  force  on  a  sphere  of  moist  clay 
made  to  revolve  rapidly  ?  Describe  the  experiment  with  the  apparatus  represented 


SECOND   LAW   OF   MOTION. 


41 


81.  The  centrifugal  force,  acting  as  just  described,  is  supposed  to  have 
given  the  earth  its  present  form.  The  matter  of  which  our  planet  is  com- 
posed seems  at  one  time  to  have  been  soft,  and  under  a  rapid  rotary  motion, 
before  becoming  solid,  it  swelled  out  at  the  equator  and  became  depressed  at 
the  poles.  The  earth  thus  became  an  oblate  spheroid,  the  distance  from  pole 
to  pole  being  about  26  miles  less  than  the  equatorial  diameter. 

Second  ]Law  of  Motion/ 

82.  A  given  force  always  prod\ 
whether  the  body  on  which  it  acts  is 
whether  it  is  acted  on  by  that  force 
the  same  time. 

'  The  earth,  as  it  turns  on  its  axis,  carries  al 
its  surface  with  great  velocity  from  west  to  east; 
force  acting  on  any  object  OR  the  surface  causes  it  to  mo^e 
in  the  same  direction,  and  with  the  same  rapidity,  as  if  the    —  * 
earth  were  at  rest. 

Let  a  stone  be  dropped  from  the  mast-head  of  a  vessel,  and  it  will  fall  at 
the  bottom  of  the  mast,  whether  the  vessel  moves  or  is  at  rest. 

A  person  sitting  in  a  wagon  throws  up  an  orange  and  catches  it  in  his 
hand,  whether  the  wagon  is  moving  or  not. 

83.  SIMPLE  MOTION. — Mo- 
tion produced  by  a  single  force 
is  called  Simple  Motion. 

84.  RESULTANT  MOTION. — 
Motion  produced  by  the  joint 
action  of  more  than  one  force 
is  called  Resultant  Motion. 

Resultant  motion  is  illustrated  with 
the  apparatus  represented  in  Fig.  32. 
The  ball  C  is  placed  on  a  square  frame 
between  two  upright  wires,  on  each  of 

which  a  ball  slides  so  as  to  strike  C  when  it  descends.  Let  the  ball  A  drop, 
and  it  will  drive  C  to  D ;  this  is  an  example  of  simple  motion.  Let  the  ball 
B  drop,  and  it  will  drive  C  to  E  ;  this,  also,  is  simple  motion.  Let  A  and  B 


Fig.  32. 


•• 


in  Fig.  31.  81.  What  is  supposed  to  have  been  the  effect  of  the  centrifugal  force  on 
the  form  of  the  earth  ?  How  does  the  equatorial  diameter  of  the  earth  compare  with 
the  distance  from  pole  to  pole  ?  82.  What  is  the  Second  Law  of  Motion  ?  Give  some 
familiar  illustrations  of  this  law.  88.  What  is  Simple  Motion  ?  84.  What  is  Eesult- 
aut  Motion?  Describe  the  apparatus  with  which  resultant  motion  is  illustrated. 


42 


MECHANICS. 


drop  at  the  same  instant,  and  they  will  drive  C  to  F;   this  is  resultant 

motion. 

Fig.  33.  85.  "We  have  an  example  of  resultant  motion  in 

a  boat  (see  Fig.  33)  which  a  person  attempts  to 
row  north  across  a  river,  while  the  tide  carries  it 
to  the  east.  Each  force  produces  the  same  effect 
as  if  it  acted  alone ;  and  the  boatman,  when  he 
has  crossed  the  river,  will  find  himself  neither  due 
north  nor  due  east  of  the  point  from  which  he 

A  D         started,  but  north-east  of  it. 

If,  in  addition  to  the  boatman's  efforts  and  the  tide,  the  wind  should  blow, 

this  also  will  produce  its  full  effect ;  and  the  boat  will  exhibit  a  resultant 

motion  produced  by  the  joint  action  of  the  three  forces. 

86.  THE  PARALLELOGRAM  OF  MOTION. — If  Figures  32 
and  33  be  examined,  it  will  be  seen  that  a  body  acted  on 
by  two  forces  moves  in  a  diagonal  direction,  between  the 
lines  in  which  they  would  separately  propel  it. 

In  Fig.  33,  the  boatman,  starting  at  A,  would  row  his  boat  to  B  ;  the  tide 
in  the  same  time  would  carry  it  to  D.  When  both  act,  to  get  the  direction 
of  the  boat  and  the  point  it  would  reach,  we  must  draw  the  other  sides  of  the 
parallelogram,  B  C,  D  C ;  the  diagonal  A  C  will  then  show  the  course  of  the 
boat,  and  its  extremity  C  the  point  it  would  reach. 

87.  If  the  two  forces  are  equal,  the  body  will  move  in 
the  diagonal  of  a  square,  that  is,  directly  between  the  lines 
in  which  they  would  carry  it.     If  one  is  greater  than  the 

other,  the  parallelogram  must  be  constructed 
accordingly. 

Let,  for  instance,  the  force  used  by  the  boatman  be  twice 
as  great  as  that  of  the  tide.  Then  by  the  time  he  would  reach 
B,  the  tide  would  have  carried  his  boat  one-half  of  that  dis- 
tance, to  D.  Completing  the  parallelogram,  as  in  Fig.  34,  and 
drawing  the  diagonal  A  C,  we  find  that  under  the  joint  action 
of  these  forces  the  boat  would  reach  C, 


Fig.  34 


Third  Law  of  motion. 

88.  Action  is  the  force  which  one  body  exerts  on  an- 
other subjected  to  its  operation. 


85.  How  may  resultant  motion  be  illustrated  in  the  case  of  a  boat?  86.  How  does  a 
tody  acted  on  by  two  forces  move  ?  Illustrate  this  with  Figure  33.  87.  If  the  two 
forces  are  equal,  how  will  the  body  move  ?  If  the  forces  are  unequal,  how  will  it 
move?  Apply  this  principle  in  Fig.  34  88.  What  is  Action?  What  is  Ecaction? 


THIBD   LAW    OF   MOTION.  43 

Reaction  is  the  counter-force  which  the  body  acted 
upon  exerts  on  the  body  acting. 

The  third  Law  of  Motion  is  as  follows  : — Reaction 
is  always  equal  to  Action^  and  opposite  to  it  in  direc- 
tion. 

89.  Examples  of  Action  and  Reaction.— We  strike  an  egg  against  a  table ; 
the  table  reacts  on  the  egg  with  the  same  force  and  in  the  contrary  direction, 
breaking  its  shell.     We  push  a  wagon  forward,  and  feel  the  reaction  in  the 
resistance  it  offers.   A  bird,  when  flying,  strikes  the  air  downward  blows  with 
its  wings ;  the  air  reacts  upward  and  supports  the  bird.     A  rower  pulls  his 
oar  against  the  water ;  the  water  reacts  and  drives  the  boat  in  the  opposite 
direction.    A  boy  fires  a  gun ;  the  exploding  powder  carries  forward  the 
ball,  but  the  air  thus  struck  reacts  on  the  gun  and  causes  it  to  recoil  against 
the  boy's  shoulder.     Two  boats  of  equal  weight,  A  and  B,  are  connected  with 
a  rope  :  a  man  in  A  pulls  the  rope ;  action  and  reaction  being  equal,  not  only 
will  the  boat  B  move  towards  him,  but  the  boat  A,  which  he  is  in,  will  move 
with  the  same  velocity  towards  B. 

90.  It  is  reaction  that  kills  a  person  who  falls  from  a  height  on  a  hard 
pavement.     Another,  falling  the  same  distance,  lights  on  a  feather  bed,  and 
receives  little  or  no  injury ;  not  because  there  is  less  reaction,  but  because  the 
reaction  is  more  gradual,  and  therefore  his  body  does  not  receive  so  great  a 
shock.    On  the  same  principle,  if  a  steamboat  in  making  her  landing  is  likely 
to  strike  violently  against  the  dock,  the  force  of  the  collision  is  deadened  and 
the  boat  saved  from  damage  by  interposing  a  coil  of  rope,  or  some  other  sub- 
stance softer  than  wood. 

Hence  also  a  bullet,  which  would  penetrate  a  board,  will  not  go  through 
a  soft  cushion,  its  motion  being  gradually  and  not  instantaneously  opposed 
by  the  reaction  of  the  cushion.  A  person  may  catch  a  very  heavy  stone 
without  being  hurt,  if  he  allows  his  hand,  the  instant  he  catches  it,  to  be  car- 
ried in  the  direction  in  which  the  stone  was  moving,  and  thus  makes  the  re- 
action gradual. 

91.  Reaction  often  nullifies  action.  This  was  the  case 
with  the  man  who  tried  to  raise  himself  over  a  fence  by 
pulling  at  the  straps  of  his  boots.  Tug  as  he  might,  he 
found  that  all  the  upward  impulse  he  could  give  himself 
was  counterbalanced  by  an  equally  strong  downward  im- 
pulse, and  that  his  utmost  efforts  could  not  reverse  the  law 


What  is  the  Third  Law  of  Motion  ?  89.  Give  some  familiar  illustrations  of  the  third 
law  of  motion.  90.  What  is  the  effect  of  reaction  on  a  person  falling  from  a  height  on 
B  hard  pavement?  "What  is  the  effect,  if  the  person  falling  lights  on  a  feather  bed  ? 
What  causes  the  difference  ?  Give  another  instance  of  gradual  reaction.  How  may  a 
person  catch  a  very  heavy  stone  without  being  hurt?  91.  What  is  often  the  effect 


MECHANICS. 


of  nature  —  that  action 
and  reaction  are  equal  in 
force  and  opposite  in  di- 
rection. 

We  read  of  another  man  no 
less  ingenious,  who  rigged  a  huge 
bellows  in  the  stern  of  his  sail- 
boat, that  he  might  always  be 
able  to  make  a  fair  wind.  On 
trying  the  experiment,  he  found 
that  with  all  his  blowing  he  could 
not  move  the  boat  an  inch  ;  for 
the  reaction  of  the  air  on  the  bel- 
lows kept  her  back  as  much  as  its  action  on  the  sail  tended  to  move  her  for- 
ward. 

92.  ACTION  AND  REACTION  IN  NON-ELASTIC  AND  ELASTIC 
BODIES. — Action  and  reaction  are  always  equal,  but  they 
are  exhibited  differently  in  non-elastic  and  elastic  bodies. 
This  difference  is  shown  with  suspended  balls  of  soft  clay 
and  ivory,  the  latter  of  which  are  elastic,  while -the  former 
are  the  reverse. 

Fig.   36   represents  two    clay    or  non- 
1      elastic  balls.     A  is  raised  and  allowed  to 
fall.     If  it  met  with  no  resistance,  it  would 
rise  to  about  the  same  height  on  the  oppo- 
site side.     But,  encountering  B,  it  imparts 
1  •*    to  it  a  portion  of  its  motion,  and  both  move 
on  together,  as  shown  in  Fig.  37,  though  only 
half  as  far  as  A  would  have  gone  alone.     The 
reaction  of  B  is  clearly  equal  to  the  action  of 
A  ;  for  the  latter  loses  just  as  much  motion  as 
the  former  gains. 

If  the  two  balls  be  of  ivory,  or  any  other 
highly  elastic  substance,  A  will  impart  the  whole  of  its  mo- 
tion to  B,  and  remain  stationary  after  striking ;  while  B,  as 

<>f  reaction?  What  humorous  instance  is  given  of  the  nullifying  effect  of  reaction  ? 
State  the  case  of  the  man  with  the  sail-boat.  92.  In  what  two  classes  of  bodies  ara 
fcction  and  reaction  differently  exhibited  ?  How  is  this  difference  shown  ?  What 
£oes  Fig.  36  represent?  Show  the  effect  of  action  and  reaction  in  these  non-elastio 


Fig.  36. 


Fig.  37. 


ACTION   AND   REACTION. 


45 


Fig.  38. 


A0 


Fie.  39. 


shown  in  Fig.  38,  will  swing  to  the  same 
height  that  A  would  have  reached  if  unre- 
sisted.  Here  again  the  reaction  of  B,  which 
brings  A  to  rest,  is  evidently  equal  to  the 
action  of  A,  which  sets  B  in  motion. 

93.  Fig.  39  affords  a  further  illustration  of  action  and  reaction  in  elastic 
bodies.  Five  ivory  balls  are  suspended  by  strings  of  equal  length,  so  as  to 
fall  in  front  of  a  graduated  arc,  with  the  aid  of 
which  the  distance  they  move  can  be  observed. 
Let  the  first,  A,  be  drawn  out  and  allowed  to 
fall.  It  will  impart  all  its  motion  to  the  second, 
and  by  the  reaction  of  the  latter  will  be  brought 
to  rest.  In  like  manner,  the  second  imparts  its 
motion  to  the  third,  and  is  kept  at  rest  by  reac- 
tion ;  and  so  with  the  third  and  the  fourth.  The 
fifth,  B,  finally  receives  the  motion ;  and,  there 
being  in  this  case  no  reaction  to  stop  it,  it  flies 
off  to  the  same  height  from  which  A  started. 

94.  REFLECTED  MOTION. — Reflected  Motion  is  the  mo- 
tion of  a  body  turned  from  its  course  by  the  reaction  of 
another  body  against  which  it  strikes.  A  ball  rebounding 
from  a  wall  against  which  it  has  been  thrown,  affords  an 
example  of  Reflected  Motion. 

If  a  body  possessing  little  or  no  elasticity  be  thrown  against  a  wall,  it  will 
rebound  but  a  short  distance,  if  at  all.  We  find  the  most  striking  instances 
of  reflected  motion  in  the  most  elastic  bodies.  Every  boy  knows  that  an 
India  rubber  ball  will  bound  higher  than  one  made  of  yarn,  and  that  a  yarn 
ball  will  bound  higher  than  one  stuffed  with  cotton. 

95.  When  a  ball  is  thrown  perpendicularly  Fig.  40. 

against  another  body,  it  rebounds  in  the  same 
line  towards  the  hand  from  which  it  was  thrown. 
Thus,  in  Fig.  40,  if  a  ball  be  thrown  from  F  against 
the  surface  B  C  so  as  to  strike  it  perpendicularly 
at  A,  it  will  return  in  the  line  A  F.  If  thrown 
from  D,  however,  it  will  glance  off  on  the  other 
side  of  the  perpendicular,  at  the  same  angle,  to  E. 
If  D  were  nearer  the  perpendicular,  the  line  A  E  would  also  be  nearer  to  it ; 
if  it  were  farther  from  the  perpendicular,  AE  would  be  farther  in  proportion. 

balls.  What  does  Fig.  33  represent  ?  93.  Describe  the  apparatus  represented  in  Fig. 
39,  and  tell  how  it  operates.  94.  What  is  Reflected  Motion  ?  Give  an  example. 
What  bodies  exhibit  reflected  motion  most  strikingly  ?  95.  When  a  ball  is  thrown 
perpendicularly  against  another  body,  how  does  it  rebound  ?  When  thrown  so  as  to 
make  an  angle  with  the  perpendicular,  how  will  it  rebound  ?  Illustrate  this  with 


46  MECHANICS. 

96.  The  angle  D  AF  in  Fig.  40,  made  by  the  body  in 
its  forward  course  with  the  perpendicular  at  the  point  of 
contact,  is  called  the  Angle  of  Incidence. 

The  angle  E  A  F,  made  by  the  body  in  its  backward 
course  with  the  same  perpendicular,  is  called  the  Angle  of 
Reflection. 

The  great  law  of  reflected  motion  is  as  follows  : — The 
Angle  of  Reflection  is  always  equal  to  the  Angle  of  Inci- 
dence. 


CHAPTER  Y. 

MECHANICS    (CONTINUED).. 

GRAVITY. 

97.  TERRESTRIAL  GRAVITY. — When  a  stone  is  let  go,  we 
all  know  that  it  does  not  fly  up  in  the  air  or  move  sideways, 
but  falls  to  the  ground.     This  is  owing,  as  already  men- 
tioned, to  a  universal  property  of  matter.     The  stone  and 
the  earth  mutually  attract  each  other  ;  but  the  earth,  being 
vastly  superior  in  size,  draws  the  stone  to  itself,  or  in  other 
words,  causes  it  to  fall. 

The  tendency  of  bodies,  when  unsupported,  to  approach 
the  earth's  surface,  is  called  Terrestrial  Gravity,  or  simply 
Gravity. 

98.  GRAVITATION. — Attraction  is  universal.     It  is  not 
confined  to  things  on  and  about  the  earth's  surface,  but 
extends  throughout  space,  millions  of  miles,  and  is  in  fact 
the  great  agent  by  which  the  heavenly  bodies  are  kept 
moving  in  their  respective  spheres.    The  earth  as  certain- 
ly attracts  the   planet  Uranus,   at  the  vast  distance    of 
1,828,000,000  miles,  as  it  does  the  falling  stone. 

Figure  40.    96.  What  is  the  Angle  of  Incidence  ?    "What  is  the  Angle  of  Reflection  ? 
What  is  the  great  law  of  reflected  motion  ? 

97.  When  a  stone  is  let  go,  what  does  it  do  ?    To  what  is  this  owing  ?    What  is 
meant  by  Terrestrial  Gravity  ?    93.  What  is  Gravitation  ?    How  far  does  gravitation 


GRAVITATION.  47 

The  attraction  subsisting  between  the  heavenly  bodies 
is  called  Gravitation. 

To  Sir  Isaac  Newton  the  world  owes  the  great  discovery  of  the  law  of 
Universal  Gravitation.  Galileo  had  investigated  the  subject  of  terrestrial 
gravity  (A.  D.  1590),  but  he  did  not  imagine  that  any  similar  force  existed 
beyond  the  neighborhood  of  the  earth.  Kepler  advanced  a  step  nearer  the 
truth,  and  spoke  of  gravitation  as  acting  from  planet  to  'planet;  still  he  did 
not  conceive  of  its  having  any  effect  on  the  planetary  motions.  This  discov- 
ery, one  of  the  most  important  that  modern  science  has  achieved,  was  re- 
served for  the  mighty  genius  of  Newton.  Sitting  in  his  orchard  one  day 
(A.  D.  1666),  he  observed  an  apple  fall  from  a  bough.  This  simple  circum- 
stance awakened  a  train  of  thought.  Gravity,  he  knew,  was  not  confined  to 
the  immediate  surface  of  the  earth.  It  extended  to  the  greatest  heights  with 
which  man  was  acquainted  ;  why  might  it  not  reach  out  into  space?  Why 
not  affect  the  moon  ?  Why  not  actually  cause  her  to  revolve  around  the 
earth  ?  To  test  these  speculations,  Newton  at  once  undertook  a  series  of  la- 
borious calculations,  which  proved  that  the  attraction  of  gravitation  is  uni- 
versal ;  that  it  determines  the  orbits  and  velocities  of  the  planets,  causes  the 
inequalities  observed  in  their  motions,  produces  tides,  and  has  given  its 
present  shape  to  the  earth. 

99.  Three  facts  have  been  established  respecting  gravi- 
tation : — 

1.  Gravitation  acts  instantaneously.    Were  a  new  body 
created  in  space  1,000  miles  from  the  earth,  its  attraction 
would  be  felt  at  the  sun  just  as  soon  as  at  the  earth,  though 
the  one  would  be  95,000,000  miles  off,  and  the  other  only 
1,000. 

2.  Gravitation  is  not  lessened  by  the  interposition  of 
any  substance.     The  densest  bodies  offer  no  obstacle  to  its 
free  action.     Were  a  body  placed  on  the  other  side  of  the 
moon,  it  would  be  attracted  by  the  earth  just  as  much  as 
if  the  moon  were  not  between  them. 

3.  Gravitation  is  entirely  independent  of  the  nature  of 
matter.     All  substances  that  contain  equal  amounts  of  mat- 
ter attract  and  are  attracted  by  any  given  body  with  equal 

extend  ?  Give  an  example.  By  whom  was  the  law  of  Universal  Gravitation  discor- 
«red  ?  What  advance  had  been  made  towards  it  by  Galileo  ?  What,  by  Kepler  ? 
Give  an  account  of  the  circumstances  and  reasoning  that  led  Newton  to  this  discov- 
ery. What  was  proved  by  his  calculations  ?  99.  What  is  the  first  fact  that  has  been 
established  respecting  gravitation?  Give  an  example.  What  is  the  second  fact? 
Give  an  example.  What  is  the  third  fact  ?  What  evidence  is  there  of  this  ?  100.  What 


48  MECHANICS. 

force.  The  action  of  the  sun  is  found  to  be  the  same  on 
all  the  heavenly  bodies. 

100.  DIRECTION  OF  GRAVITY. — If  a  piece  of  lead  sus- 
pended by  a  string  be  left  free  to  move,  it  will  point  to- 
wards the  earth.  This  is  the  case  in  all  parts  of  the  globe. 
Now,  as  the  earth  is  round,  it  follows  that  at  two  opposite 
points  of  its  surface,  the  plummet,  or  plumb-line  (as  this 
Fig.  41.  suspended  lead  is  called), will 

point  in  opposite  directions. 
This  will  be  seen  from  the 
relative  positions  of  A  and 
B,  C  and  D,  in  Fig.  41.  The 
lead,  therefore,  has  no  ten- 
dency to  fall  in  any  particu- 
lar direction  as  such,  but 
takes  all  directions  according 
to  the  part  of  the  earth's 
surface  which  it  is  near.  The 
universal  law  is,  that  it  must  point  towards  the  centre  of 
the  earth. 

It  is  not  because  any  peculiar  attractive  power  resides  in  the  centre  that 
a  falling  body  tends  towards  that  point ;  but  because,  in  a  sphere,  this  is  the 
result  of  the  attraction  of  all  the  particles.  The  particles  on  one  side  attract 
the  falling  body  as  much  as  those  on  the  other ;  and  consequently  it  seeks  a 
point  between  them. 

No  two  plummets  suspended  in  different  places  have  exactly  the  same  di- 
rection, for  the  lines  in  which  they  hang  would  meet  at  the  centre  of  the 
earth.  At  short  distances,  however,  the  difference  of  direction  is  so  slight  as 
to  be  imperceptible,  and  the  plummets  seem  to  point  the  same  way. 

101.  It  follows  that  up  and  down  are  relative  and  not  absolute  terms. 
What  is  up  to  a  person  in  New  York,  is  down  to  a  ship  a  few  miles  south-west 
of  Australia.  If  a  person  in  a  standing  position  at  New  York  were  to  be 
carried  in  a  straight  line  through  the  earth  to  its  centre,  and  on  in  the  same 
direction  to  the  opposite  side  of  the  earth,  he  would  come  out  in  the  Indian 
Ocean  south-west  of  Australia,  but  would  find  himself  on  his  head  instead  of 
his  feet.  His  head,  which  at  New  York  pointed  up,  would  now  point  down. 

is  a  piece  of  lead  suspended  by  a  string  called  ?  How  does  the  plummet  always 
point?  On  what  does  the  absolute  position  of  the  plummet  depend?  Why  does  a 
falling  body  tend  towards  the  centre  of  the  earth?  What  is  said  of  the  difference  of 
direction  in  plummets  suspended  in  different  places  ?  101.  What  is  said  of  the  terms 
up  and  down  t  Exemplify  this.  What  is  the  real  meaning  of  up  and  doivn  T  Why 


LAWS   OF    GRAVITY.  49 

Down,  therefore,  simply  means  towards  the  centre  of  the  earth,  and  up  away 
from  the  centre. 

This  explains  what  the  unreflecting  are  sometimes  puzzled  to  account  for, 
. — why  persons  and  things  on  the  side  of  .the  earth  opposite  to  them  do  not 
fall  off.  Regarding  themselves  as  on  the  upper  side,  they  can  not  see  what 
keeps  those  on  the  under  side  from  being  precipitated  into  space.  But  really 
there  is  no  under  side.  All  things  are  alike  drawn  towards  the  centre ;  all 
are  kept  on  the  earth's  surface  by  the  same  force  of  gravity. 

102.  LAWS  FOB  THE  FORCE  OF  GRAVITY. — The  force  of 
gravity  (and  the  term  is  here  used  in  its  widest  sense,  in- 
cluding gravitation)  depends  on  two  things, — 1.  Amount 
of  matter ;  2.  Distance, — according  to  the  following  laws  : 

1.  The  force  of  gravity  increases  as  tJie  amount  of  mat- 

ter increases. 

2.  The  force  of  gravity  decreases  as  the  square  of  the 

distance  increases. 

103.  According  to  the  first  law,  if  the  sun  contained 
twice  as  much  matter  as  it  now  does,  it  would  attract  the 
earth  with  twice  its  present  force ;  if  it  contained  three 
times  as  much  matter,  with  three  times  its  present  force ; 
&c.     Observe,  we  say  if  it  contained  twice  as  much  matter, 
not  if  it  were  twice  as  large /  for  it  might  be  twice  its 
present  size,  and  yet  so  rare  as  to  contain  less  matter  and 
attract  less  strongly  than  it  now  does.     If  there  were  two 
heavenly  bodies,  the  one  of  iron  and  the  other  of  cork,  the 
latter,  though  twice  as  large  as  the  former,  would  have  less 
attraction  because  it  would  contain  less  matter. 

As  already  remarked,  the  earth  is  so  much  larger  than  the  bodies  near 
its  surface  that  it  is  not  perceptibly  affected  by  their  attraction.  Even  if  a 
ball  500  feet  in  diameter  were  placed  in  the  atmosphere  500  feet  from  tho 
earth's  surface,  the  earth,  being  580  million  million  times  greater  than  the 
ball,  would  draw  the  latter  to  itself,  while  it  would  advance  to  meet  it,  less 
than  one  ninety-six-thousand-millionth  of  an  inch — a  distance  so  small  that  it 
«an  not  be  appreciated. 

The  sun  is  800  times  greater  than  all  the  planets  put  together.  It  is  on 
account  of  this  enormous  amount  of  matter  that  its  attraction  is  felt  by  the 
most  remote  bodies  of  the  solar  system  at  a  distance  of  many  millions  of  miles. 

do  not  objects  on  the  under  side  of  the  earth  fall  off?  102.  On  what  does  the  force  of 
gravity  depend  ?  Eepeat  the  two  laws  of  gravity.  103.  Explain  the  first  law.  Why 
is  not  the  earth  perceptibly  affected  by  the  attraction  of  bodies  near  its  surface  ?  Give 
an  example.  Why  is  the  attraction  of  the  sun  so  great  ?  What  would  be  its  effect 

3 


50  MECHANICS. 

A  man  carried  to  the  surface  of  the  sun  would  be  so  strongly  attracted  by  its 
immense  mass  that  he  would  be  literally  crushed  by  his  own  weight. 

104.  According  to  the  second  law,  if  the  sun  were  twice 
as  far  from  the  earth  as  it  now  is,  it  would  attract  the  latter 
with  but  J  of  its  present  force  ;  if  three  times  as  far,  with 
i ;  if  four  times  as  far,  with  Jg-,  &c.    So,  if  two  equal  masses 
were  situated  respectively  5,000  miles  and  10,000  miles  from 
the  earth's  centre,  the  nearer  would  be  attracted  not  twice, 
but  4  times,  as  strongly  as  the  more  distant. 

105.  All  bodies  on  the  earth's  surface,  however  small, 
attract  each  other  with  greater  or  less  force  according  to 
their  masses  and  distance.     This  attraction,  in  most  cases, 
is  absorbed  in  the  far  greater  attraction  of  the  earth,  and 
consequently  can  not  be  perceived.     In  the  case  of  moun' 
tains,  however,  it  is  so  strong  as  to  have  a  sensible  effect  on 
plummets  suspended  at  their  base.     Instead  of  pointing  di- 
rectly towards  the  centre  of  the  earth,  a  plumb-line  in  such 
a  position  is  found  to  incline  slightly  towards  the  mountain. 

106.  WEIGHT. — When  a  body  is  supported  or  prevented 
from  following  the  impulse  of  gravity,  it  presses  on  that 
which  supports  it,  more  or  less  strongly  according  to  the 
force  with  which  it  is  attracted.     This  downward  pressure 
is  called  its  Weight. 

"Weight  is  simply  the  measure  of  a  body's  gravity,  and  is  proportioned  to 
the  amount  of  matter  contained.  A  ball  of  iron  is  heavier  than  a  ball  of  cork 
of  equal  size,  because  it  contains  more  matter. 

Weight  being  nothing  more  than  the  measure  of  the  force  with  which 
bodies  are  drawn  towards  the  earth,  it  follows  that,  if  the  earth  contained 
twice  as  much  matter  as  it  now  does,  they  would  have  twice  their  present 
weight ;  if  it  contained  three  times  as  much  matter,  three  times  their  present 
weight,  &c. 

107.  Weight  above  and  below  the  Eartltfs  Surface. — 
Since  the  weight  of  a  body  is  the  measure  of  its  gravity, 
and  since  gravity  decreases  as  the  square  of  the  distance 
from  the  earth's  centre  increases,  it  follows  that  bodies  be- 

•n  a  man  carried  to  its  surface  ?  104  Illustrate  the  second  law  with  an  example. 
105.  Why  is  not  attraction  exhibited  between  small  bodies  on  the  earth's  surface  ? 
How  is  a  plummet  suspended  near  the  base  of  a  mountain  affected  ?  106.  "What  is 
Weight?  To  what  is  weight  proportioned?  If  the  earth  contained  twice  as  much 
matter  as  it  now  does,  how  would  the  weight  of  objects  on  its  surface  compare  witk 


WEIGHT  ABOVE  THE  EARTH'S  SURFACE. 


51 


come  lighter  in  the  same  proportion  as  they  are  taken  up 
from  the  earth's  surface.  A  mass  of  iron  which  at  the 
earth's  surface  weighs  a  thousand  pounds,  taken  up  to  a 
height  of  4,000  miles,  would  weigh  only  250  of  such  pounds, 
or  one-fourth  as  much  as  before. 


Fig.  42. 


20,000  miles 
6  times  surface  distance 


16,000  mile* 
4  times  surface  distance 


12,000  mile* 
8  times  surface  diatanM 


8,000  mflei 
'  Twice  surface  distance 


•wrfaoe  distance 


40  rounds 

l/25  surface  weigh* 


«Va 

1/16  surface  weigh. 


m'/9  pound. 
l/9  surface  weight 


250  pounds 

l/4  surface  weight 


1,000  pounds 
Surface  weight 


The  reason  of  this  is  clear.  The  earth 
being  about  8,000  miles  through,  from  its 
centre  to  its  surface  is  4,000  miles;  and 
from  its  centre  to  a  point  4,000  miles 
above  its  surface,  is  8,000  miles.  4,000  is 
to  8,000  as  1  to  2  ;  but  the  weight  at  the 
surface  would  not  be  to  the  weight  4,000 
miles  above  the  surface  as  2  to  1,  but  as 
the  squares  of  these  numbers,  4  to  1. 
Hence,  if  it  would  weigh  1,000  pounds  at 
the  surface,  it  would  weigh  only  y4  as 
much,  4,000  miles  above  the  surface.  For 
the  same  reason,  it  would  weigh  1/9  of 
1,000  pounds  at  a  distance  of  8,000  miles 
from  the  surface ;  l/u,  at  a  distance  of 
12,000  miles ;  Vas>  at  a  distance  of  16,000 
miles,  &c.  These  results  are  exhibited 
in  Fig.  42. 

At  small  elevations,  the  weight  which 
an  object  loses  amounts  to  but  little.  Four 
miles  above  the  earth's  surface,  a  body 
weighing  1,000  pounds  would  become  only 
two  pounds  lighter.  Raised  to  a  height 
of  240,000  miles,  the  distance  of  the  moon 
from  the  earth,  its  weight  would  be  re- 
duced to  less  than  five  ounces.  / 

108.  If  we  could  go  from  the 
Surface  of  the  earth  to  the  cen- 
tre, we  should  find  a  given  object  weigh  lesa  and  less  as  we 
advanced.  The  moment  we  descended  beneath  the  surface, 
we  would  leave  particles  of  matter  behind  us,  and  the  at- 
traction of  these  would  act  in  a  direction  exactly  opposite 
to  gravity. 

their  present  weight  ?  107.  What  is  said  of  the  weight  of  bodies  taken  up  from  the 
earth's  surface  ?  What  would  1,000  pounds  of  iron  weigh,  4,000  miles  above  the 
earth's  surface  ?  Show  the  reason  of  this.  What  is  said  of  the  loss  of  weight  at  small 
elevations?  Four  miles  above  the  surface,  how  much  would  a  body  weighing  1,000 
pounds  lose  ?  WThat  would  be  its  weight,  240,000  miles  from  the  earth  ?  108.  If  we 


52 


MECHANICS. 


Thus,  in  Fig.  43,  let  C  represent  the  centre  of  the  earth,  and  0  any  ohjecl 
beneath  the  surface.    All  the  particles  below  the  line  A  B  attract  0  down- 
Fig.  43.  Fig.  44. 


ward,  but  all  above  that  line  attract  it  upward,  and  thus  diminish  its 
weight. 

At  the  centre  of  the  earth  (see  Fig.  44)  no  object  would  weigh  any  thing. 
There  would  be  as  many  particles  above  the  line  DE  as  below  it;  and  0,  be- 
ing equally  attracted  on  all  sides,  would  have  no  weight. 

109.  All  bodies  carried  below  the  earth's  surface  would,  therefore,  become 
lighter  as  they  approached  the  centre.  Their  weight  at  any  given  number 
of  miles  below  the  surface  may  be  found  as  follows : — 

For  1  mile  below,  take  f  ${}T  of  the  surface  weight. 

For        2  miles,  take  f|f  ^  of  the  surface  weight. 

For     100  miles,  take  f  £££  of  the  surface  weight. 

For  1,000  miles,  take  '^~  of  the  surface  weight,  &c. 

110.  Law  of  Weight. — From  the 
above  principles  the  following  law  of 
weight  is  deduced: — All  objects  weigh 
the  most  at  the  surface  of  the  earth: 
ascending  from  the  surface^  their 
weight  diminishes  as  the  square  of 
their  distance  from  the  centre  in- 
creases ;  descending  towards  the  cer* 
tre,  their  weight  diminishes  as  their 
distance  from  the  surface  increases. 

Fig.  45    shows  the  operation  of 
this  law  in  the  case  of  an  object  weigh- 
ing 1,000  pounds  at  the  earth's  surface. 

could  go  from  the  surface  of  the  earth  to  the  centre,  what  would  we  find  respecting 
the  weight  of  a  given  body?  What  is  the  reason  of  this  decrease?  Illustrate  this 
with  Fig.  43.  What  would  all  objects  \reigh  at  the  centre  ?  Show  the  reason  of  this 
with  Fig.  44.  109.  IIow  may  we  find  the  weight  of  a  given  body  one  mile  below  the 


WEIGHT   IN   DIFFERENT   LATITUDES. 


53 


111.  Weight  at  different  Parts  of  the  Earth?  s  Surface. 
— The  weight  of  a  body  differs  at  different  parts  of  the 
earth's  surface.     A  mass  of  lead,  for  instance,  that  weighs 
1,000  pounds  at  the  poles,  will  weigh  only  995  such  pounds 
at  the  equator. 

112.  This  is  owing  to  two  causes : — 

1.  The  equatorial  diameter  is  about  26£  miles  longer 
than  the  polar  diameter ;  and  therefore  an  object  at  the 
equator  is  farther  from  the  centre  and  less  strongly  at- 
tracted than  at  any  other  point. 

2.  The  centrifugal  force,  as  shown  in  §  79,  is  greatest 
at  the  equator,  and  therefore  counterbalances  more  of  the 
downward  attraction  there  than  at  any  other  part  of  the 
surface,  making  the  weight  less.     It  has  been  computed, 
that,  if  the  earth  revolved  17  times  as  fast  as  it  now  does, 
the  centrifugal  force  at  the  equator  would  counterbalance 
gravity  entirely,  and  thus  deprive  all  bodies  of  weight.     If 
the  earth's  velocity  were  further  increased,  all  things  at  the 
equator  would  be  thrown  off  into  space. 

113.  The  general  effect  of  gravity  is 
to  draw  bodies  towards  the  earth ;  but 
sometimes  it  causes  them  to  rise.     A 
balloon,  for    instance,  mounts   to    the 
clouds.     This  is  because  it  contains  less 
matter  than  a  mass  of  air  of  the  same 
bulk,  or,  as  we  say  briefly,  it  is  lighter 
than  air.     Hence  the  air,  acted  on  more 
strongly  by  gravity  than  the  balloon,  is 
drawn  towards  the  earth  under  the  lat- 
ter, which  is  thus  caused  to  rise. 

For  the  same  reason,  smoke  ascends. 
So,  if  a  flask  of  oil  be  uncorked  at  the 


Fig.  46. 


A  BALLOON. 


earth's  surface  ?  Tv/o  miles  ?  A  hundred  miles  ?  A  thousand  miles  ?  110.  Eepeat 
the  law  of  weight.  111.  What  is  said  of  the  weight  of  a  body  at  different  parts  of  the 
earth's  surface ?  Give  an  example.  112.  To  what  causes  is  this  owing?  What  would 
be  the  result,  if  the  earth  revolved  on  its  axis  with  seventeen  times  its  present  velo- 
city ?  113.  Show  how  gravity  sometimes  causes  a  body  to  rise.  Give  some  illustra- 


54  MECHANICS. 

bottom  of  a  pail  of  water,  the  water  will  be  drawn  down 
below  the  oil,  and  force  the  latter  to  the  top. 

Falling  Bodies. 

114.  VELOCITY  OP  FALLING  BODIES. — If  a  feather  and 
a  cent  be  dropped  from  a  height  at  the  same  time,  the  cent 
will  reach  the  ground  some  seconds  before  the  feather. 
This  fact  Aristotle  and  his  successors  explained  by  teaching 
that  the  velocity  of  falling  bodies  is  proportioned  to  their 
weight ;  that  a  body  of  two  pounds,  for  instance,  would 
reach  the  ground  in  just  half  the  time  required  by  a  body 
weighing  one  pound.     Galileo  was  the  first  to  correct  this 
error  (about  A.  D.  1590).     He  held  that  the  velocity  of  fall- 
ing bodies  is  independent  of  their  weight,  and  that,  if  no 
other  force  than  gravity  acted  on  them,  all  objects  dropped 
at  the  same  time  from  the  same  height  would  reach  the 
ground  at  the  same  instant. 

So  startling  a  proposition  was  at  once  condemned  by  the  learned  men  of 
the  day ;  but  Galileo,  convinced  of  the  truth  of  his  position,  challenged  his 
opponents  to  a  trial. 

The  leaning  tower  of  Pisa  [pe'-zah],  Italy,  was  chosen  as  the  scene  of  the 
experiment,  and  multitudes  flocked  to  witness  it.  Two  balls  were  produced, 
one  of  which  weighed  exactly  twice  as  much  as  the  other,  and  after  being 
examined,  to  prevent  the  possibility  of  deception,  at  a  given  signal  they  were 
dropped.  In  breathless  anxiety  the  crowd  awaited  the  result,  doubting  not 
that  it  would  confound  the  bold  youth  of  six-and-twenty  years,  who  had  dared 
to  oppose  not  only  the  sages  of  his  own  time,  but  also  the  established  opin- 
ion of  centuries  and  the  great  master  Aristotle  himself.  To  their  amazement, 
the  bold  youth  was  right ;  the  balls  reached  the  earth  at  the  same  instant. 
Unable  to  credit  their  own  senses,  again  and  again  they  repeated  the  experi- 
ment, but  each  time  with  the  same  result.  This  triumph,  though  it  awakened 
the  jealousy  of  his  defeated  rivals,  and  cost  Galileo  his  place  as  professor  of 
mathematics  in  the  university  of  Pisa,  established  the  fact  that  gravity  causes 
all  bodies  to  descend  with  equal  rapidity,  without  reference  to  their  weight,  and 
that  all  apparent  differences  are  caused  by  some  other  agency. 

115.  RESISTANCE  OF  THE  AIR. — The  cause  of  the  differ- 


tions.  114.  If  a  feather  and  a  cent  be  dropped  at  the  same  time,  which  will  reach  the 
ground  first  ?  How  did  Aristotle  explain  this  fact  ?  What  was  Galileo's  opinion  on 
the  subject  ?  How  was  his  theory  received  by  the  learned  men  of  the  day  ?  Give  an 
wcount  of  the  trial  that  was  made  at  Pisa.  What  fact  was  established  by  the  experi- 


KESISTANCE    OF   THE   AIR. 


55 


Fig.  47. 


\\ 


fence  of  velocity  in  a  falling  feather  and  a  falling  cent  is  the 
Resistance  of  the  Air. 

This  resistance  is  proportioned  to  the  extent  of  surface 
which  the  falling  body  presents  to  the  air.  The  surface, 
indeed,  may  be  so  extended  that  gravity  can 
hardly  overcome  the  air's  resistance;  thus,  gold 
may  be  beaten  into  a  leaf  so  thin  that  it  will  be 
exceedingly  slow  in  its  descent,  floating  for  a  time 
in  the  air. 

116.  That  the  resistance  of  the  air  causes  the  difference  of  ve- 
locity exhibited  by  falling  bodies,  may  be  proved  in  two  ways : — • 

1.  A  piece  of  paper,  a  sheet  of  gold-leaf,  or  a  feather,  with  its 
surface  extended,  floats  slowly  downward ;  roll  it  into  a  compact 
mass,  and  it  will  descend  rapidly  like  a  stone. 

2.  Remove  the  air  from  a  high  glass  tube  (see  Fig.  47)  by 
means  of  an  instrument  called  the  air-pump,  to  be  described 
hereafter.     Then,  from  an  apparatus  provided  for  the  purpose, 
drop  a  feather  and  a  cent  simultaneously,  and  they  will  reach  the 
bottom  at  precisely  the  same  instant.    Let  in  the  air  and  drop 
them,  and  the  feather  will  be  several  seconds  longer  than  the  cent 
in  reaching  the  bottom. 

117.  The  Parachute. — It  is  the  resistance  of  the 
air  that  enables  a  person  to  descend  in  safety  from 

a  balloon  at  great  heights  above  the  earth's  surface.     A 
parachute,  which  spreads  open  like  a  large  umbrella,  is  sus- 
pended beneath  the  balloon.    Hav-  Fig.  48. 
ing  taken  his  position  in  the  bas- 
ket-shaped car  hanging  beneath, 
the  aerial  voyager  fearlessly  de- 
taches himself  from  the  balloon ; 
for,  though  he  is  borne  downward 
by  gravity,  the  force  of  his  fall  is 
so  broken  by  the  resistance  which 
the  air  offers  to  the  extended  sur- 
face of  the  parachute  that  he  incurs              A  PARACHTTTE- 

ment  ?  What  was  its  result  to  Galileo  ?  115.  What  causes  the  difference  of  velocity  in 
a  falling  feather  and  a  falling  cent  ?  To  what  is  the  resistance  of  the  air  proportioned  ? 
How  may  the  air's  resistance  almost  be  made  to  counterbalance  gravity  ?  Give  an 
illustration.  116.  Prove  in  two  ways  that  the  resistance  of  the  air  causes  the  differ- 
ence of  velocity  in  falling  bodies.  117.  How  is  a  person  enabled  to  descend  safely 


56  MECHANICS. 

little  danger.  To  ensure  the  safety  of  a  common-sized 
man,  a  parachute  must  be  at  least  22  feet  across.  Fig.  48 
represents  a  parachute;  Fig.  46  shows  it  attached  to  a 
balloon. 

118.  LAW  OF  FALLING  BODIES. — "We  have  found  that 
all  bodies  acted  on  solely  by  gravity  fall  to  the  earth  with 
the  same  velocity.  It  is  evidently  an  accelerated  velocity  ; 
for  gravity,  which  first  causes  the  motion,  continues  acting. 
In  other  words,  gravity  gives  a  falling  body  a  certain  ve- 
locity in  the  first  second  of  its  descent ;  still  forcing  it 
downward,  it  increases  that  velocity  in  the  following  sec- 
ond ;  and  so  on  till  it  reaches  the  earth. 

To  find  the  exact  spaces  passed  over  in  successive  sec- 
onds, and  the  velocity  at  any  given  point  of  the  descent, 
was  formerly  exceedingly  difficult,  on  account  of  the  rapid- 
ity with  which  falling  bodies  move,  and  the  want  of  conve- 
niences for  experimenting  on  them.  Even  the  greatest 
perpendicular  heights  were  inadequate  to  the  purpose,  as 
a  falling  body  would  reach  their  base  in  a  few  seconds. 
These  difficulties  are  now  removed  by  an  ingenious  appa- 
ratus, called,  after  its  inventor,  Atwood's  Machine. 

119.  Atwood's  Machine. — Atwood's  Machine  is  represented  in  Fig.  49.  It 
consists  of  a  pillar,  G,  about  six  feet  high,  surmounted  by  a  horizontal  plate, 
J  K ;  from  which  to  the  base  of  the  stand  extends  a  perpendicular  graduated 
scale,  C  L,  divided  into  feet,  inches,  and  tenths  of  an  inch.  The  plate  J  K 
supports  a  vertical  wheel,  D,  the  axis  of  which,  that  it  may  revolve  as  far 
as  possible  without  friction,  rests  on  four  other  wheels,  a,  J,  c,  d  (d,  being 
behind  the  rest,  is  not  seen  in  the  figure).  A  and  B  are  equal  weights,  con- 
nected by  a  cord,  which  passes  over  the  wheel  D.  F  is  a  pendulum  which 
vibrates  once  in  a  second ;  and  I  is  a  dial-plate  and  index  (like  the  face  and 
hand  of  a  clock)  for  marking  seconds. 

B,  having  exactly  the  same  weight  as  A,  just  counterbalances  it.  Now 
attach  to  A  a  small  weight  equal  to  one  sixty-third  of  the  combined  weight 
of  A  and  B.  This  slight  addition  causes  A  to  descend ;  but  as  A  descends, 
B  of  course  ascends;  and  as  neither  A  nor  B,  being  counterbalanced 

from  a  balloon  at  a  great  height?  Describe  the  process.  How  large  must  a  parachute 
be  for  a  common-sized  man  ?  118.  With  what  sort  of  velocity  must  falling  bodies  de- 
Bcend?  Why  so?  What  made  it  difficult  formerly  to  ascertain  the  velocity,  &c.,  of 
falling  bodies  ?  What  apparatus  is  now  employed  for  this  purpose  ?  119.  Describe 
Atwood's  Machine  from  the  plate.  Show  its  mode  of  operation.  How  does  this  ma- 


ATWOOD'S   MACHINE. 


each  by  the  other,  has  any  gravity, 
the  gravity  of  the  small  weight  at- 
tached to  A,  which  sets  them  in  mo- 
tion, must  be  divided  into  64  equal 
parts.  Hence  A  with  the  added 
weight  is  64  times  as  long  in  descend- 
ing as  it  would  be  if  dropped  freely 
in  the  air,  and  the  experimenter  thus 
has  an  opportunity  of  observing  its 
velocity  at  different  points,  and  as- 
certaining the  relative  distances  pass- 
ed over  during  the  successive  beats 
of  the  pendulum.  The  distances  pass- 
ed over  in  the  first,  the  secorid,  the 
third,  and  the  fourth  second,  &c.,  bear 
the  same  relation  to  each  other,  as 
if  the  bodies  were  falling  freely  in 
space.  The  velocity,  moreover,  hav- 
ing been  greatly  diminished,  the  re- 
sistance of  the  air  becomes  so  slight 
that  it  need  not  be  taken  into  calcu- 
lation. 

120.  It  is  found  with  Atwood's 
Machine,  that,  calling  the  distance 
traversed  in  the  1st  second  1,  that 
traversed  in  the  2d  will  be  3 ;  that 
in  the  3d,  5 ;  that  in  the  4th,  7  ;  and 
so  on  in  the  series  of  odd  numbers. 
The  velocity  at  the  end  of  the  1st  sec- 
ond will  be  a  mean  between  1  and  3, 
or  2  ;  at  the  end  of  the  2d,  it  will  be 
a  mean  between  3  and  5,  or  4  ;  at  the 
end  of  the  3d,  6 ;  at  the  end  of  the 
4th,  8  ;  and  so  on  in  the  series  of  even 
numbers. 

In  1  second  a  falling  body  descends 
16Vi2  feet ;  therefore,  according  to  the 
results  obtained  with  Atwood's  Ma- 
chine, it  has  a  velocity  at  the  end  of  the 
1st  second  of  twice  16Yi2  feet,  or  32ys 

chine  aid  the  experimenter  ?  120.  What  is 
found  with  Atwood's  Machine,  respecting 
the  distances  traversed  in  successive  sec- 
onds ?  What  is  the  relative  velocity  at  the 
tod  ef  successive  seconds  ?  How  far  do«e 
a  body  fall  in  the  first  second  ?  According 


58  MECHANICS. 

feet,  per  second.  In  the  second  second  it  descends  3  times  16»/ia'feet,  or  48>/4 
feet,  and  at  its  termination  has  a  velocity  of  4  times  16  Via  feet,  or  64'/3  feet, 
per  second.  In  the  third  second,  it  descends  5  times  IGVia  feet,  or  805/ia  feet, 
and  at  its  termination  has  a  velocity  of  6  times  IG1/^,  or  9GYa  feet,  per  sec- 
ond, &c. 

Now,  as  to-the  whole  space  passed  through  in  any  given  time.  In  1  sec- 
ond, it  will  be  16Viafeet;  in  2  seconds,  by  addition  (16Vi2  +  48y4),  64»/3  feet; 
in  3  seconds,  (16y12  +  4Sy4  +  805/ia)  1443/4feet;  in  4  seconds,  (16y12-f4Sy4 
+  805/ia+  H27/i2)  257Vs,  and  so  on. 

121.  These  results  are  summed  up  in  the  following  rules : — 

Rule  1. — To  find  the  space  through  which  a  falling  body 
passes  during  any  second  of  its  descent,  multiply  16^  feet 
by  that  one  in  the  series  of  odd  numbers  which  corresponds 
with  the  given  second. 

Example.  How  far  will  a  stone  fall  in  the  tenth  second  of  its  descent  ? — 
The  series  of  odd  numbers  is  1,  3,  5,  7,  9, 11,  13, 15, 17, 19,  &c.  The  tenth 
is  19;  16Via  multiplied  by  19  gives  3057/12.— Answer,  30oT/i2  feet. 

Rule  2. — To  find  the  velocity  of  a  falling  body  at  the 
termination  of  any  second  of  its  descent,  multiply  16TV  feet 
by  that  one  in  the  series  of  even  numbers  which  corresponds 
with  the  given  second. 

Example.  What  is  the  velocity  of  a  stone  that  has  been  falling  ten  sec- 
onds ?— The  series  of  even  numbers  is  2,  4,  6,  8,  10,  12,  14,  16,  18,  20.  The 
tenth  is  20;  IG^ia  multiplied  by  20  gives  3212/3. — Answer,  3212/3  feet  per 
second. 

Rule  3. — To  find  the  whole  space  passed  through  by  a 
falling  body,  multiply  16TV  feet  by  the  square  of  the  given 
number  of  seconds. 

Example.  How  far  will  a  stone  fall  in  10  seconds  ? — Squaring  10  gives 
100 ;  16Vi2  multiplied  by  100  gives  l,60Sy3.— Answer,  l,60Sy3  feet. 

122. — BODIES  THROWN  DOWNWARD. — These  rules  apply 
to  bodies  acted  on  by  gravity  alone.  If  a  body  is  thrown 
downward,  the  force  with  which  it  is  thrown  must  also  be 
taken  into  calculation. 

Thus,  if  a  stone  be  cast  from  a  height  with  a  force  that  would  propel  it  50 

to  the  results  obtained  with  Atwood's  Machine,  how  far  will  it  fall  in  successive  sec- 
onds, and  what  will  be  its  velocity  at  the  end  of  each  ?  121.  Eepeat  Eule  1,  for  find- 
ing the  space  traversed  by  a  falling  body  during  any  second  of  its  descent.  Apply 
this  rule  in  the  given  example.  Eepeat  Eule  2,  for  finding  the  velocity  of  a  falling 
tody.  Apply  this  rale  in  an  example.  Eepeat  Eule  3,  for  finding  the  whole  distance 
traversed  by  a  falling  body.  Give  an  example.  122.  To  what  bodies  do  these  rules 


BODIES   THROWN   DOWNWARD.  59 

feet  in  a  second,  then  in  the  tenth  second,  instead  of  falling  3057/ia  feet,  as  in 
the  example  under  Rule  1,  it  would  fall  50  feet  farther, — that  is  3557/ia  feet. 
Its  velocity  at  the  end  of  the  tenth  second  would  likewise  be  obtained  by  add- 
ing 50  feet  per  second  to  the  velocity  obtained  in  the  example  und*er  Rule  2 : 
3213/3  +  50  =  37P/3- — To  obtain  the  whole  space  passed  through,  add  to  the 
result  obtained  by  Rule  3,  the  distance  traversed  in  consequence  of  the  velo- 
city originally  imparted.  A  body  thrown  downward  with  a  velocity  of  50  feet 
per  second,  would,  without  any  aid  from  gravity,  pass  through  500  feet  in  10 
seconds.  Adding  this  to  1,608  l/a  feet,  the  distance  through  which  gravity 
alone  causes  a  body  to  fall  in  10  seconds,  we  have  2,108  Ys  feet  for  the  whole 
distance  traversed  in  that  time  by  a  body  thrown  downward  with  a  velocity 
of  50  feet  per  second. 

123.  In  the  above  examples,  no  allowance  is  made  for 
the  resistance  of  the  air.     But  even  the  bodies  most  favor- 
ably shaped  for  falling  feel  the  effects  of  this  resistance. 
Experiments  in  St.  PauVs  Cathedral,  London,  show  that  in 
4i  seconds  a  body  falls  272  feet ;  whereas,  according  to  the 
principles  stated  above,  it  should  fall  325  feet.     This  differ- 
ence, which  amounts  to  nearly  one-sixth  of  the  whole  dis- 
iance,  is  owing  principally  to  the  resistance  of  the  air. 

124.  As  the  velocity  of  a  falling  body  increases  32|  feet 
every  second,  it  does  not  take  long  for  it  to  acquire  a  tre- 
mendous speed ;  and,  as  the  striking  force  is  proportioned 
to  the  weight  multiplied  into  the  square  of  the  velocity, 
it  is  clear  that  even  a  small  body,  falling  any  considerable 
distance,  may  become  a  very  powerful  agent.     Hence  the 
disastrous  effects  of  hail-stones,  which  have  been  known  to 
injure  cattle  and  break  through  the  roofs  of  houses,  and 
which  prove  so  destructive  to  the  vineyards  in  parts  of 
Southern  Europe  that  the  fields  have  to  be  protected  from 
their  visitations. 

125.  ASCENDING  BODIES. — As  a  falling  body  increases 
in  velocity  32£  feet  every  second  of  its  descent,  so  an  as- 
cending body,  being  acted  on  by  the  same  force,  loses  a 

apply  ?  If  a  body  is  thrown  from  a  height,  what  must  enter  into  the  calculation  ?  If 
a  stone  were  thrown  down  with  a  force  that  would  propel  it  50  feet  in  a  second,  how 
far  would  it  fall  in  the  tenth  second  ?  What  would  be  its  velocity  at  the  end  of  the 
tenth  second  ?  What  would  be  the  whole  distance  traversed  in  ten  seconds  ?  123.  For 
what  must  allowance  be  made  in  applying  these  rules  ?  How  great  a  difference  does 
the  resistance  of  the  air  occasion  ?  124.  How  are  the  disastrous  effects  of  hail-stones 
accounted  for  ?  125.  What  is  said  of  the  velocity  of  an  ascending  body  ?  How  may 


60  MECHANICS. 

like  amount,  and  will  at  last  be  brought  to  rest.  The  num- 
ber of  seconds  during  which  it  will  continue  to  rise  is  found 
by  dividing  the  number  of  feet  per  second  with  which  it 
starts  by  32£. 

The  height,  therefore,  which  an  ascending  body  reaches, 
depends  on  the  force  with  which  it  is  projected  upward  ; 
and,  were  there  no  air  to  resist  its  progress,  it  would  al- 
ways reach  such  a  height  as  it  would  have  to  fall  from  in 
order  to  acquire  the  velocity  with  which  it  started.  The 
spaces  traversed  and  the  velocities  attained  during  succes- 
sive seconds  would  be  the  same  in  the  ascent  as  in  the  de- 
scent, only  reversed  in  order. 

Thus,  if  projected  upward  with  a  velocity  of  3212/3  feet  per  second,  a  ball 
unresisted  by  the  air  would  continue  to  rise  10  seconds ;  because,  to  attain  a 
velocity  of  321%  feet  from  a  state  of  rest,  it  would  have  to  fall  10  seconds. 
In  the  tenth  second  of  its  ascent,  it  would  pass  through  the  same  distance  as 
in  the  first  second  of  its  descent,  16yia  feet;  in  the  ninth  second  of  its  as- 
cent, the  same  as  in  the  second  of  its  descent,  4sy4  feet;  in  the  eighth  second 
of  its  ascent,  the  same  as  in  the  third  of  its  descent,  &c. 

126.  According  to  the  principle  just  stated,  a  rifle-ball,  shol  vertically  up- 
ward, would  descend  on  whatever  it  struck  with  the  same  force  that  it  had 
when  originally  discharged.  But  it  does  not  do  so,  on  account  of  the  resist- 
ance of  the  air.  This  resistance  prevents  the  ball  from  rising  as  high  as  it 
otherwise  would  do  by  about  one-sixth  of  the  whole  distance  (see  §  123),  and 
in  its  descent  it  again  loses  nearly  one-sixth.  The  whole  loss  thus  amounts 
to  nearly  one-third  of  the  velocity,  leaving  a  little  over  two-thirds  remaining. 
Now,  to  find  the  proportion  between  the  striking  force  of  the  ball  when  origi- 
nally projected  and  its  striking  force  on  returning  to  the  same  point,  we  must 
square  two-thirds.  This  gives  four-ninths  ;  and  thus  we  find  that  the  ball,  on 
returning  to  the  surface,  strikes  an  object  with  less  than  half  the  effect  which 
it  has  immediately  on  being  discharged — a  result  borne  out  by  facts. 

Projectiles. 

.1.2 7.  A  Projectile  is  a  body  thrown  through  the  air.  An 
arrow  discharged  from  a  bow,  a  bullet  from  a  gun,  a  stone 
from  the  hand,  are  all  Projectiles. 

•we  find  the  number  of  seconds  that  an  ascending  body  will  continue  to  rise  ?  "Were 
it  not  for  the  resistance  of  the  air,  how  great  a  height  would  a  body  projected  upward 
attain  ?  What  is  said  of  the  spaces  traversed  and  the  velocities  attained  during  suc- 
cessive seconds  ?  Exemplify  this  in  the  case  of  a  ball  thrown  upward  with  a  velocity 
6f821f  feet  per  second.  126.  According  to  this  principle,  with  what  force  would  a 
ball  shot  vertically  upward  descend  on  an  object  ?  Does  it  do  so  ?  Explain  the  rea- 


PROJECTILES. 


61 


Every  projectile  is  acted  on  by  three  forces : — 

1.  The  force  by  which  it  was  thrown. 

2.  Gravity,  which  constantly  impels  it  towards  the  earth. 

3.  The  resistance  of  the  .air,  which  tends  to  bring  it  to 

rest. 

128.  PATH  OF  A  PROJECTILE. — A  projectile  maybe  thrown 
with  such  force  as  to  be  borne  some  distance  in  a  straight 
line,  without  having  its  direction  sensibly  altered  by  grav- 
ity or  the  air's  resistance ;  as  in  the  case  of  a  cannon-ball. 
When,  however,  its  velocity  diminishes,  the  joint  action  of 
these  forces  causes  it  to  move  in  a  line  more  or  less  resem- 
bling the  curve  called  the  pa-rdb'-o-la.  The  less  the  pro- 
jectile force,  the  sooner  does  the  body  deviate  from  a 
straight  line  to  a  curve. 

Fig.  50. 


Fig.  50  shows  the  path  of  a  stone  thrown  obliquely  from  the  hand.  The 
propelling  force  sends  it  in  a  straight  line  to  A,  and  would  take  it  on  in  the 
same  direction  to  B,  were  it  not  that,  as  soon  as  its  velocity  becomes  suffi- 
ciently diminished,  gravity  and  the  air's  resistance  give  it  a  circular  motion 
to  C,  and  finally  bring  it  to  the  earth  at  D. 

129.  If  thrown  straight  up,  a  projectile  will  descend 
in  the  same  line  in  which  it  ascended.  If  discharged  hori- 
zontally from  a  height,  it  will  describe  a  curve  which  varies 

BOH  -why  it  does  not.  12T.  What  is  a  Projectile?  Give  examples.  Enumerate  the 
forces  by  which  every  projectile  is  acted  on.  128.  "When  a  projectile  is  discharged 
with  great  force,  what  is  its  direction  for  a  time  ?  When  its  velocity  diminishes,  how 
ffoee  it  move  ?  What  projectiles  deviate  soonest  from  a  straight  line  ?  Illustrate  th» 
path  of  a  projectile  with  Fig.  50.  129.  If  thrown  straight  up,  how  does  a  projectila 


62  MECHANICS. 

in  form  according  to  the  velocity  originally  imparted.  The 
greater  this  velocity,  the  greater  the  distance  the  projectile 
will  pass  through ;  but,  whatever  the  distance  traversed,  it 
will  always  reach  the  ground  in  precisely  the  same  time  that 
it  would  take  to  fall  to  the  earth  from  the  height  at  which 
it  was  discharged. 

Thus,  in  Fig.  51,  we  have  a  cannon 
planted  on  a  tower  at  such  a  height 
that  it  would  take  four  seconds  for  a 
ball  to  fall  from  it  to  the  ground. 
Dropped  from  the  cannon's  mouth,  in 
the  first  second  a  ball  would  reach 
A ;  in  the  next,  B  :  in  the  third,  C ;  and 
in  the  fourth,  D.  Fired  from  the  can- 
non, and  acted  on  by  the  projectile 
force  alone,  it  would  in  one,  two,  three, 
and  four  seconds,  successively  reach 

35,  F,  G,  and  H.  When  both  forces  act,  the  ball  will  move  in  the  dotted  line, 
reaching  at  the  end  of  the  successive  seconds  the  points  I,  J,  K,  and  L.  The 
ball  fired  from  the  cannon  will  touch  the  ground  at  L  at  precisely  the  same 
instant  that  the  ball  dropped  from  it  will  strike  the  ground  at  D. 

130.  The  resistance  of  the  air,  which  is  but  slight  when  a  body  moves 
slowly  through  it,  becomes  a  powerful  agent  as  the  velocity  of  the  body  in- 
creases. A  cannon-ball,  fired  with  a  velocity  of  2,000  feet  in  a  second,  would 
go  24  miles  before  gravity  alone  would  stop  it ;  whereas,  when  opposed  by 
the  air's  resistance,  as  well  as  gravity,  it  goes  but  3. 

131.  A  projectile  reaches  a  greater  height  and  remains 
longer  in  the  air,  when  thrown  straight  upward,  than  when 
thrown  in  any  other  direction. 

132.  RANDOM. — The  Random,  or  Range,  of  a  projectile 
is  the  distance  in  a  straight  line  between  the  points  at  which 
it  begins  and  ceases  to  move. 

When  thrown  perpendicularly  upward,  a  projectile  re- 
turns to  the  point  from  which  it  started,  and  the  random 
is  naught.  The  more  its  course  deviates  from  the  perpen- 
dicular the  greater  the  random  becomes,  until  it  is  thrown 

descend  ?  If  discharged  horizontally  from  a  height,  what  kind  of  a  line  does  a  pro- 
jectile describe  ?  What  projectiles,  so  discharged,  will  traverse  the  greatest  distance  ? 
How  long  will  it  take  projectiles  discharged  horizontally  from  a  height  to  reach  the 
ground  ?  Explain  these  principles  with  Fig.  51.  130.  In  what  case  does  the  resist- 
ance of  the  air  become  a  very  powerful  agent  ?  Show  this  in  the  case  of  a  cannon 
ball.  131.  In  what  direction  must  a  projectile  be  thrown,  to  attain  the  greatest 


GUNNERY.  63 

at  an  angle  of  somewhat  less  than  40  degrees,  from  which 
point  it  again  diminishes.  Were  it  not  for  the  resistance 
of  the  air,  a  projectile  would  have  the  greatest  random 
when  thrown  at  an  angle  of  45  degrees. 

Figure  52  shows  the 
course  of  projectiles 
thrown  at  different  angles. 
The  ball  which  leaves  the 
cannon's  mouth  at  an  an- 
gle of  about  37  degrees 
will  be  the  only  one  to  hit 
the  vessel.  The  two  balls 
fired  at  a  greater  and  a 
less  angle  will  fall  short 
of  it. 

133.  GUNNEEY. — The  laws  relating  to  projectiles  form 
the  basis  of  the  science  of  Gunnery.      The  artilleryman 
must  know  just  at  what  angle  to  elevate  his  gun,  and  how 
great  an  allowance  to  make  for  gravity  and  the  *air's  re- 
sistance. 

134.  Military  projectiles  are  discharged  with  the  aid  of 
gunpowder.     This  is  a  solid,  which  by  the  application  of  a 
spark  is  instantaneously  converted  into  a  highly  elastic 
fluid,  and  in  that  form  expands  to  many  times  its  previous 
bulk.     This  sudden  expansion,  confined  within  a  cannon, 
finds  vent  at  its  mouth,  and  with  such  force  as  to  impart 
great  velocity  to  a  ball  or  other  missile. 

Who  invented  gunpowder  can  not  be  ascertained.  It  was  known  many 
centuries  before  the  Christian  era  to  the  Chinese,  who  used  it  for  levelling 
hills,  blasting  rocks,  and  also,  as  the  remains  of  ancient  pieces  of  ordnance 
indicate,  for  military  purposes.  Other  eastern  nations  appear  to  have  been 
acquainted  with  its  use  at  an  early  date.  Roger  Bacon,  the  celebrated  Eng- 
lish philosopher,  in  a  work  written  about  1270  A.  D.,  alludes  to  it  as  a  well 
known  composition.  Fifty  years  later,  Berthold  Schwartz,  a  Prussian  monk, 

height?  132.  What  is  the  Eandom  of  a  projectile?  What  is  the  random  of  a  pro- 
jectile thrown  perpendicularly  upward  ?  At  what  angle  must  a  projectile  be  dis- 
charged, to  have  the  greatest  random  ?  What  would  be  the  angle,  were  it  not  for 
the  resistance  of  the  air  ?  Explain  Fig.  52.  133.  What  science  is  based  on  the  laws 
of  projectiles?  134.  How  are  military  projectiles  discharged?  Explain  the  mode  in 
which  a  projectile  is  discharged  with  gunpowder.  By  whom  was  gunpowder  invent- 
<?d  ?  To  whom  was  it  early  known  ?  What  English  philosopher  alluded  to  it,  and 
When?  What  Prussian  monk  investigated  its  properties?  Where  and  when  were 


64 


MECHANICS. 


Fig.  53. 


investigated  its  properties ;  he  has  by  some  been  called  its  inventor,  as  Ba- 
con has  by  others.  The  first  that  we  hear  of  cannon's  being  used  in  war  is 
at  the  battle  of  Cressy,  between  the  French  and  English,  A.  D.  1346. 

135.  As  the  striking  force  of  a  body  increases  with  the 
square  of  its  velocity,  the  pieces  of  ordnance  used  in  attack- 
ing a  fort  are  so  charged  as  to  give  the  balls  the  greatest 
possible  velocity.  In  naval  engagements,  on  the  other 
hand,  no  greater  velocity  is  desired  than  will  just  plant  the 
balls  in  the  enemy's  hull ;  for  thus,  imparting  the  whole  of 
their  motion  to  the  ship,  they  give  it  a  greater  shock,  and 
do  more  damage  by  splintering  its  tim- 
bers, than  if  they  have  sufficient  velocity 
to  carry  them  completely  through. 

136.  THE  BALLISTIC  PENDULUM. — Sev- 
eral methods  have  been  tried  for  measur- 
ing the  velocity  of  cannon  and  musket 
balls.  One  is  to  suspend  the  piece  from 
which  the  ball  is  fired  and  measure  its  re- 
coil ;  action  and  reaction  being  equal,  this 
recoil  is  proportioned  to  the  force  with 
which  the  bah1  is  discharged.  Another 
method  is  by  means  of  an  instrument  call- 
ed the  Ballistic  Pendulum,  represented  in 


Fig.  53. 


THE    BALLISTIC    PEN- 
DULUM. 


From  a    cross-piece,  A,  on  a  stout  framework,  a 
heavy  block  of  wood,  B,  is  suspended,  in  such  a  way  as 

to  move  freely  backward  and  forward.  A  ball  fired  into  this  block  will 
drive  it  back  to  a  distance  proportioned  to  the  ball's  velocity.  This  distance 
is  measured  by  a  ribbon,  C  D,  attached  to  the  lower  end  of  the  pendulum, 
which  is  drawn  through  an  orifice  in  the  cross-piece  E  as  the  block  is  carried 
back.  The  weight  of  the  block,  the  distance  it  is  driven,  and  the  weigh*  <>f 
the  ball  being  known,  the  velocity  of  the  ball  can  be  determined. 

137.  It  is  found  by  experiments  with  the  ballistic  pen- 
dulum that  the  greatest  velocity  that  can  be  given  to  a 
cannon-ball  is  a  little  over  2,000  feet  in  a  second:.  To  make 

cannon  first  used  in  war  ?  135.  How  are  pieces  of  ordnance  charged  for  attacking  a 
fort?  How  in  naval  engagements,  and  with  what  object?  136.  "What  methods  have 
fce«n  tried  for  measuring  the  velocity  of  balls  ?  Describe  the  BalL^ic  Pendulum. 
l3t.  What  is  the  greatest  velocity  that  cau  be  given  to  a  cannon-ball  ?  "VTiat  te  <aid 


THE   PENDULUM. 


65 


a  piece  carry  the  greatest  distance,  it  must  be  charged  with 
a  certain  amount  of  powder,  which  is  not  uniform,  but  va- 
ries even  in  different  pieces  of  the  same  size.  A  larger 
charge  is  not  only  useless,  but  dangerous,  as  it  may  burst 
the  gun. 

The  longer  the  barrel  of  a  gun,  the  greater  is  the  velo- 
city imparted  to  the  ball;  but  its  random  is  thus  only 
slightly  increased,  and,  for  various  reasons,  great  length  is 
now  regarded  as  a  positive  disadvantage. 

The  Pendulum. 

138.  A  Pendulum  Fig.  54 
consists    of    a    heavy 

ball  suspended  in  such 
a  way  as  to  swing  to  and 
fro.  Fig.  54  represents 
a  Pendulum. 

If  raised  on  one  side  and 
let  go,  the  ball  of  the  pendu- 
lum, B,  will  be  carried  down 
by  gravity  with  such  force  as 
to  rise  by  its  inertia  to  the 
same  height  on  the  opposite 
side.  From  this  point  it  will  """-£ 

again  fall  and  rise  on  the  other 
side;  and,  if  no  other  force 
than  gravity  operated,  it  would 
keep  on  rising  and  falling  for- 
ever. The  friction  at  the  point  of  suspension,  however,  and  the  resistance 
of  the  air,  are  constantly  tending  to  check  its  motion ;  and  the  consequence 
is  that  it  swings  each  time  a  less  distance,  and  finally  comes  to  rest. 

139.  When  swinging  to  and  fro,  a  pendulum  is  said  to 
vibrate  /  and  the  portion  of  a  circle  through  which  it  moves 
is  called  its  arc.     In  Fig.  54,  CD  is  the  arc  of  the  pendu- 
lum A  B. 

140.  LAWS    OP  VIBRATION. — First  Law. — The   vibra- 

of  the  amount  of  powder  to  be  used  for  a  charge?  What  is  the  effect  of  lengthening 
the  barrel  of  a  gun  ?  138.  Of  what  does  a  Pendulum  consist  ?  What  takes  place  when 
a  pendulum  is  raised  on  one  side  and  iet  go  ?  What  causes  it  finally  to  come  to  rest? 
139.  When  is  a  pendulum  said  to  vibrate  f  What  is  meant  by  the  arc  of  a  pendulum  ? 


"' 


B 

THE  PENDULUM. 


66  MECHANICS. 

tions  of  a  given  pendulum  are  performed  in  very  nearly  the, 
same  time,  whether  it  moves  through  longer  or  shorter  arcs. 

Thus,  in  Fig.  54,  if  the  pendulum  A  B  were  raised  only  to  E,  it  would  be 
as  long  in  swinging  from  E  to  F  as  from  C  to  D.  The  shorter  the  arc,  there- 
fore, the  slower  its  motion.  It  is  on  this  principle  that  a  swing,  when  first 
set  in  motion,  goes  very  slowly,  but  increases  in  velocity  as  it  is  pushed 
higher  and  higher. 

141.  Second  Law.  —  The  vibrations  of  pendulums  of 
different  length  are  performed  in  different   times  /    and 
their  lengths  are  proportioned  to  the  squares  of  their  tjimes 
of  vibration. 

One  pendulum  vibrates  in  2  seconds,  another  in  4  Then  the  latter  will 
be  four  times  as  long  as  the  former  ;  because  they  will  be  to  each  other  as 
the  square  of  2  is  to  the  square  of  4,  —  that  is,  as  4  is  to  16.  Hence,  to  have 
its  time  of  vibration  doubled,  a  pendulum  must  be  made  4  times  as  long  ;  to 
have  it  tripled,  9  times  as  long  ;  to  have  it  quadrupled,  16  times  as  long,  &c. 
A  pendulum,  to  vibrate  only  once  in  a  minute,  would  have  to  be  60  times  60, 
that  is  3,600,  times  as  long  as  one  that  vibrates  once  in  a  second,  —  or  a  little 
over  2  miles. 

Conversely,  the  times  in  which  different  pendulums  vibrate  are  to  each 
other  as  the  square  roots  of  their  length.-  If  one  pendulum  be  IS  feet  long 
and  another  4,  the  former  will  be  twice  as  long  in  vibrating  as  the  latter  ; 
for  their  times  of  vibration  are  to  each  other  as  the  square  root  of  16  is  to 
the  square  root  of  4,  —  or  as  4  to  2. 

142.  Third  Law.  —  The  vibrations  of  the  same  pendu- 
lum are  not  performed  in  the  same  time  at  all  parts  of  the 
6arth>s  surface  /  but,  being  caused  by  gravity,  differ  slight- 
ly, like  gravity,  according  to  the  distance  from  the  earths 
centre. 

On  the  top  of  a  mountain  five  miles  high,  for  instance,  a  pendulum  vibrat- 
ing seconds  would  make  10  less  vibrations  in  an  hour  than  at  the  level  of  the 
sea,  because  it  would  be  farther  from  the  earth's  centre.  At  either  pole,  a 
second-pendulum  would  make  13  more  vibrations  in  an  hour  than  at  the  equa- 
tor, because  it  is  nearer  the  centre,  the  earth  being  flattened  at  the  poles. 
Hence  the  vibrations  of  the  pendulum  afford  a  means  of  measuring  heights. 


140.  What  is  the  first  law  relating  to  the  pendulum  ?    Illustrate  this  with  Fig.  56. 

141.  "What  is  the  second  law  ?    Apply  this  law  in  an  example.    When  the  lengths  of 
different  pendulums  are  known,  how  can  we  find  the  relative  times  of  vibration  ?    If 
we  have  two  pendulums,  16  and  4  feet  long,  how  will  their  times  of  vibration  com- 
pare ?    142.  "What  is  the  third  law  ?    What  is  the  difference  in  the  number  of  vibra- 
tions in  a  second-pendulum  at  the  level  of  th3  sea  and  at  an  elevation  of  five  miles  ? 
How  would  the  number  of  vibrations  at  the  pole  compare  with  those  at  the  equator  ? 


THE  PENDULUM   APPLIED   TO   CLOCK-WORK.  67 

They  also  confirm  what  we  have  learned,  that  the  polar  diameter  of  the  earth 
is  26  i  miles  shorter  than  its  equatorial  diameter. 

In  the  latitude  of  New  York,  a  pendulum,  to  vibrate  seconds,  must  be 
about  39 Vio  inches  long;  whereas  at  Spitzbergen,  in  the  far  North,  it  must 
be  a  little  over  39 1/5,  and  at  the  equator  exactly  39  inches. 

143.  APPLICATION  OF  THE  PENDULUM  TO  CLOCK-WORK. 
— Galileo,  to  whom  science  owes  so  much,  was  the  first  to 
think  of  turning  the  pendulum  to  a  practical  use.     Observ- 
ing that  a  chandelier  suspended  from  the  ceiling  of  a  church 
in  Pisa,  when  moved  by  the  wind,  vibrated  in  exactly  the 
same  time,  whether  carried  to  a  greater  or  less  distance,  he 
at  once  saw  that  a  similar  instrument  might  be  employed 
in  measuring  small  intervals  of  time  in  astronomical  obser- 
vations. 

To  adapt  it  to  this  use,  it  was  necessary  to  invent  some 
way  of  counterbalancing  the  constant  loss  of  motion  caused 
by  friction  and  the  air's  resistance.  This  was  done  by  the 
Dutch  astronomer  Huygens  \hi'-genz\,  who  in  the  year 
1656  first  applied  the  pendulum  to  clock-work.  To  this 
great  invention  modern  astronomy  owes  its  precision  of  ob- 
servation, and  consequently  much  of  the  progress  it  has 
made. 

144.  As  a  pendulum  vibrating  seconds,  which  is  over 
39  inches  long,  would  be  inconvenient  in  clocks,  it  is  custom- 
ary to  use  one  that  vibrates  half-seconds ;  which,  according 
to  the  principles  laid  down  in  §  141,  is  one-fourth  as  long, 
or  a  little  less  than  10  inches. 

145.  At  the  same  distance  from  the  equator,  the  same 
elevation  above  the  sea,  and  the  same  temperature,  a  pen- 
dulum of  given  length  will  always  vibrate  in  exactly  the 
same  time,   and   a  clock  regulated  by  a  pendulum  will 
keep  uniform  time.    If  taken  from  the  equator  towards  the 
poles,  the  pendulum  will  vibrate  more  rapidly,  and  the  clock 

What  is  the  length  of  a  second-pendulum  at  New  York  ?  At  Spitzbergen?  At  the 
equator  ?  143.  Who  first  thought  of  turning  the  pendulum  to  a  practical  use  ?  Re- 
late the  circumstance  that  led  him  to  do  so.  To  enable  it  to  measure  small  intervals 
•of  time,  what  was  first  necessary  ?  Who  did  this,  and  thus  first  applied  the  pendu- 
lum to  clock-work  ?  144.  What  is  the  length  of  the  pendulums  generally  used  in 
tlocks  ?  145.  Under  what  circumstances  will  a  pendulum  always  vibrate  in  the  same 


68  MECHANICS. 

will  go  too  fast.  If  taken  up  a  mountain,  the  pendulum 
will  vibrate  less  rapidly,  and  the  clock  will  go  too  slow.  If 
expanded  by  the  heat  of  summer  (for  such  we  shall  here- 
after learn  is  the  effect  of  heat),  the  pendulum  will  also  vi- 
brate less  rapidly,  and  the  clock  will  go  too  slow. 

146.  THE  GKIDIKON  PENDULUM.  —  To  prevent  a  clock 
from  being  affected  by  heat  and  cold,  the  Compensation 
Pendulum  is  used. 

Fig.  55.  One  form  of  the  Compensation  Pendulum,  known  as  the  Grid- 

iron Pendulum,  is  represented  in  Fig.  55.  It  consists  of  a  frame 
of  nine  bars,  alternately  of  steel  and  brass.  These  are  so  ar- 
ranged that  the  steel  bars,  being  fastened  at  the  top,  have  to  ex- 
pand downward  ;  while  the  brass  ones,  fastened  at  the  bottom, 
expand  upward.  The  expansive  power  of  brass  is  to  that  of  steel 
as  100  to  61  ;  therefore,  if  the  length  of  the  steel  bars  is  made 
J0%i  the  length  of  the  brass  bars,  the  expansion  of  the  one  metal 
counterbalances  that  of  the  other,  and  the  pendulum  always  re- 
mains of  the  same  length.  The  steel  bars  in  the  figure  are  rep- 
resented by  heavy  black  lines  ;  the  brass  ones,  by  close  parallel 
lines. 

147.  A  clock  is  regulated  by  lengthening  or  shortening  its 
pendulum.  This  is  done  by  screwing  the  ball  up  or  down  on 
the  rod.  The  ball  is  lowered  when  the  clock  goes  too  fast,  and 
raised  when  it  goes  too  slow. 


EXAMPLES  FOE  PRACTICE. 

L  (See  Fig.  45,  and  §§  107,  109.)  What  would  be  the  weight  (that  is,  the 
measure  of  the  earth's  attraction)  of  an  iceberg  containing  40,000  tons  of 
ice,  if  raised  to  a  height  of  1,000  miles  above  the  earth's  surface? 
What  would  it  weigh  1,000  miles  beneath  the  earth's  surface  ? 

2.  A  horse  at  the  earth's  surface  weighs  1,200  pounds  ;  what  would  he  weigh 

4,000  miles  above  the  surface  ? 

How  far  beneath  the  surface  would  he  have  to  be  sunk,  to  have  the 
same  Weight? 

3.  A  Turkish  porter  will  carry  800  pounds  ;  how  many  such  pounds  could  he 

carry,  if  he  were  placed  half  way  between  the  surface  and  the  centre  of 
the  earth,  and  retained  the  same  strength  ?  —  Ans.  1,600. 

How  many  such  pounds  could  he  carry,  if  elevated  4,000  miles  above 
the  surface  with  the  same  strength? 

time  ?  What  will  cause  it  to  vibrate  more  rapidly,  and  what  less  ?  146.  To  prevent 
a  clock  from  being  affected  by  heat  and  cold,  what  is  used  ?  Describe  the  Gridiron 
Pendulum.  147.  How  is  a  clock  regulated  ? 


EXAMPLES   FOR   PRACTICE.  69 

4.  "What  would  a  body  weighing  100  pounds  at  the  earth's  surface  weigh 

1,000  miles  above  the  surface  ? 
What  would  it  weigh  1,000  miles  below  the  surface  ? 

5.  "Would  an  18- pound  cannon-ball  weigh  more  or  less,  2,000  miles  above  the 

earth's  surface,  than  2,000  miles  below  it, — and  haw  much  ? 

6.  At  the  centre  of  the  earth,  what  would  be  the  difference  of  weight  between 

a  man  weighing  200  pounds  at  the  surface  and  one  weighing  100  pounds  ? 
Four  thousand  miles  above  the  surface,  what  would  be  the  difference 
in  their  weight  ? 

7.  (See  Rule  1,  §  121. — In  the  examples  that  follow,  no  allowance  is  made  for 

the  resistance  of  the  air.)  A  man  falls  from  a  church  steeple;  how  many 
feet  will  he  pass  through  in  the  third  second  of  his  descent  ? 
6.  How  far  will  a  stone  fall  in  the  twelfth  second  of  its  descent? 

9.  (.See  Rule  2,  §  121.)  How  great  a  velocity  does  a  falling  stone  attain  in  7 

seconds  ? 

10.  A  hail-stone  has  been  falling  one-third  of  a  minute;  what  is  its  velocity? 

11.  (See  Rule  3,  §  121.)  How  far  will  a  stone  fall  in  10  seconds? 

12.  How  far  will  a  hail-stone  fall  in  one-third  of  a  minute  ? 

13.  I  drop  a  pebble  into  an  empty  well,  and  hear  it  strike  the  bottom  in  ex- 
actly two  seconds.    How  deep  is  the  well  ? 

How  many  feet  does  the  pebble  fall  in  the  first  second  of  its  descent? 
How  many,  in  the  second  ? 

What  velocity  has  the  pebble  at  the  moment  of  striking  ? 

14.  A  musket-ball  dropped  from  a  balloon  continues  falling  half  a  minute  be- 
fore it  reaches  the  earth  ;  how  high  is  the  balloon,  and  what  is  the  velo- 
city of  the  ball  when  it  reaches  the  earth  ? 

15.  What  is  the  velocity  of  a  stone  dropped  into  a  mine,  after  it  has  been  fall- 
ing 7  seconds,  and  how  far  has  it  descended  ? 

16.  (See  §  122.)  What  would  be  the  velocity  of  the  same  stone  at  the  end  of 
the  seventh  second,  if  thrown  into  the  mine  with  a  velocity  of  20  feet  in 
a  second,  and  how  far  would  it  have  descended  ? 

17.  An  arrow  falls  from  a  balloon  for  9  seconds.    How  far  does  it  fall  alto- 
gether, how  far  in  the  last  second,  and  what  velocity  does  it  attain  ? 

What  would  these  three  answers  be,  if  the  arrow  were  discharged  from 
the  balloon  with  a  velocity  of  10  feet  per  second  ? 

18.  (See  §  125.)  How  long  will  a  ball  projected  upwards  with  a  velocity  of 
1282/3  feet  per  second,  continue  to  ascend  ? 

How  great  a  height  will  it  attain  ? 

What  will  be  its  velocity,  after  it  has  been  ascending  one  second  ? 
After  two  seconds  ?    After  three  seconds  ? 

19.  How  many  seconds  will  a  musket-ball,  shot  upward  with  a  velocity  of 
2251/6  feet  in  a  second,  continue  to  ascend  ? 

How  many  feet  will  it  rise  ? 

20.  A  stone  thrown  up  into  the  air  rises  two  seconds ;  with  what  velocity  was 
it  thrown? 

21.  (See  §  141.)  How  many  times  longer  must  a  pendulum  be,  to  vibrate  only 
once  in  a  second,  than  to  vibrate  three  times  in  a  second  ? 


70  MECHANICS. 

22.  Two  pendulums  at  the  Cape  of  Good  Hope  vibrate  respectively  in  40  sec. 
onds  and  10  seconds  ;  how  many  times  longer  is  the  one  than  the  other  ? 

23.  Two  pendulums  at  New  Orleans  vibrate  in  40  seconds  and  10  seconds ; 
how  many  times  longer  is  one  than  the  other  ? 

24.  In  the  latitude  of  New  York,  a  pendulum  vibrating  seconds  is  39Vio 
inches  in  length ;  how  long  must  one  be,  to  vibrate  once  in  10  seconds  ? 
— Ans.  3,910  inches. 

How  long  must  one  be,  to  vibrate  4  times  in  a  second  at  the  same  place  ? 
— Ans.  27Vieo  inches. 

25.  At  the  equator,  a  pendulum  39  inches  long  vibrates  once  in  a  second ;  how 
long  must  a  pendulum  be,  to  vibrate  once  in  half  an  hour  at  the  same 
place  ? 

How  long  must  one  be,  to  vibrate  10  times  in  a  second  ? 

26.  At  Trinidad,  a  seconds-pendulum  must  be  about  39V50  inches  long ;  what 
would  be  the  length  of  one  that  would  vibrate  3  times  in  a  second  ? 

What  would  be  the  length  of  one  that  would  vibrate  3  times  in  a 
minute  ? 
What  would  be  the  length  of  one  that  would  vibrate  3  times  in  an  hour  ? 


CHAPTER  YI. 

MECHANICS    (CONTINUED). 

CENTRE   OF   GRAVITY. 

148.  THE  Centre  of  Gravity  of  a  body  is  that  point 
about  which  all  its  parts  are  balanced. 

The  centre  of  gravity  is  nothing  more  than  the  centre 
of  weight.  Cut  a  body  of  uniform  density  in  two,  by  a 
plane  passed  in  any  direction  through  its  centre  of  gravity, 
and  the  parts  thus  formed  will  weigh  exactly  the  same. 
The  whole  weight  of  a  body  may  be  regarded  as  concen- 
trated in  its  centre  of  gravity. 

149.  The  Centre  of  Gravity  .must  be  carefully  distin- 
guished from  the  Centre  of  Magnitude  and  the  Centre  of 
Motion. 

148.  What  is  the  Centre  of  Gravity  ?  How  may  we  divide  a  body  of  uniform 
density  into  two  parts  of  equal  weight?  Where  may  we  regard  the  whole  weight  of 
a  body  as  concentrated?  149.  From  what  must  the  centre  of  gravity  be  carefully 


CEItfTKE   OF   GEAVlTY. 


E 

The  centre  of 


150.  The  Centre  of  Magnitude  (or,  as  we  briefly  call  it, 
the  Centre)  of  a  body,  is  a  point  equally  distant  from  its 
opposite  sides. 

151.  The  Centre  of  Motion  in  a  revolving  surface  is  a 
point  which  remains  at  rest,  while  all  the  other  points  of 
the  surface  are  in  motion. 

In  all  revolving  bodies,  a  number  of  points  remain  at 
rest.  The  line  connecting  them  is  called  the  Axis  of 
Motion,  or  briefly,  the  Axis  of  the  body. 

152.  The  centre  of  gravity  may  coincide  Fig,  56. 

with  the  centre  of  magnitude  and  lie  in  the 
axis  of  motion,  but  need  not  do  so.     In 
Fig.  56,  A  represents  a  wheel  entirely  of 
wood  of  uniform  density ;  here  the  centre 
of  gravity  coincides  with  the  centre  of 
magnitude,  C,  and  both  lie  in  the  axis  of 
motion.     B  represents   the  same  wheel 
with  its  two  lower  spokes  and  part  of  the  felly  of  lead, 
magnitude,  C,  still  lies  in  the  axis,  but  the  centre  of  gravity  has  fallen 
toD. 

"When  a  body  is  of  uniform  density,  its  centre  of  gravity  coincides  with 
its  cent  -e  of  magnitude.  When  one  part  of  a  body  is  heavier  than  another, 
the  centre  of  gravity  lies  nearer  the  heavier  part. 

153.  A  line  drawn  perpendicularly  downward  from  the 
centre  of  gravity  is  called  the  Line  of  Direction.     In  Fig. 
56,  CE  and  D  E  are  the  Lines  of  Direction. 

154.  HOW  TO  FIND  THE  CENTRE  OF  GRAVITY. The  part 

of  a  body  in  which  the  centre  of  gravity  is  situated,  may  be 
found,  in  some  cases,  by  balanc- 
ing it  on  a  point.  Thus  the  cen- 
tre of  gravity  of  the  poker  rep- 
resented in  Fig.  57  lies  directly 
over  the  point  on  which  it  is 
balanced. 

155.  In   a  solid   of  regular 

distinguished?  150.  What  is  the  Centre  of  Magnitude  ?  151.  What  is  the  Centre  of 
Motion  ?  What  is  the  Axis  of  a  revolving  sphere  ?  152.  Show,  with  Fig.  56,  how  the 
centre  of  gravity  may  not  coincide  with  the  centre  of  magnitude,  or  lie  in  the  axis. 
When  does  a  body's  centre  of  gravity  coincide  with  its  centre  of  magnitude?  "When 
one  part  is  heavier  than  another,  where  docs  the  centre  of  gravity  lie  ?  153.  What 


Fig.  57. 


72 


MECIIAlsriCS. 


Fig.  58. 


shape  and  uniform  thickness  and  density,  so  thin  that  it 
may  be  regarded  as  a  mere  surface,  such  as  a  piece  of  paste- 
board, the  centre  of  gravity  may  be  found  by  ascertaining 
any  two  straight  lines  drawn  from  side  to  side  that  will 
divide  it  into  two  equal  parts.  The  point  at  which  these 
lines  intersect  is  the  centre  of  gravity.  Thus,  in  a  parallel- 
ogram, the  centre  of  gravity  is  the  point  at  which  its  two 
diagonals  intersect. 

When  such  a  surface  is  irregular  in  shape,  sus- 
pend it  at  any  point,  so  that  it  may  move  freely, 
and  when  it  has  come  to  rest,  drop  a  plumb-line 
from  the  point  of  suspension  and  mark  its  direc- 
tion on  the  surface.  Do  the  same  at  any  other 
point,  and  the  centre  of  gravity  will  lie  where  the 
two  lines  intersect. 

Thus,  suspend  the  irregular  body  represented 
in  Fig.  58  at  the  point  A ;  and,  dropping  the 
plumb-line  AB,  mark  its  direction  on  the  surface. 
Then  suspend  it  at  C  ;  drop  the  plumb-line  C  D, 
and  mark  its  direction.  The  lines  cross  at  E,  and 
there  will  be  the  centre  of  gravity. 

156.   When  two  bodies  of   equal 
weight  are  connected  by  a  rod,  the 
centre  of  gravity  will  be  in  the  centre 
of  the  rod.   When  two  bodies  of  unequal  weight  are  so  con- 
nected, the  centre  of  gravity 
_/ss\   will  be  nearer  to  the  heavier 
^^   one.     These  principles  are  il- 
lustrated in  Fig.  59,  in  which 
—  C  represents  the  centre   of 

gravity. 

157.  STABILITY  OF  BODIES. — The  Base  of  a  body  is  its 
lowest  side.     When  a  body  is  supported  on  legs,  like  a 


Fig.  59. 


is  the  Line  of  Direction  ?  154  In  some  bodies,  how  may  the  part  in  which  the  cen- 
tre of  gravity  lies  be  found  ?  155.  How  may  the  centre  of  gravity  be  found,  in  a  thip 
solid  body  of  regular  shape  and  uniform  thickness  and  density?  How  may  it  be 
found  in  such  a  solid,  when  the  shape  is  irregular  ?  Explain  the  process  with  Fig.  58. 
156.  When  two  bodies  of  equal  weight  are  connected  by  a  rod,  where  does  the  centre 
of  gravity  lie  ?  How  does  it  lie,  when  the  bodies  are  of  unequal  weight  ?  157.  Wha/ 


CENTRE   OF   GRAVITY. 


73 


Fig.  60. 


Fig.  61.         Fig.  62. 


chair,  its  base  is  formed  by  lines  connecting  the  bottoms  of 
its  legs. 

158.  When  the  line  of  direction  falls  within  the  base,  a 
body  stands ;  when  not,  it  falls. 

In  Fig.  60,  G  is  the  centre  of  grav- 
ity ;  since  the  line  of  direction,  G  P, 
falls  within  the  base,  the  body  will 
»tand.  In  Fig.  61,  the  line  of  direc- 
tion falls  exactly  at  one  extremity  of 
the  base,  and  the  body  will  be  over- 
turned by  the  slightest  force.  In  Fig.  62,  the  line  of  direction  falls  outside  of 
the  base,  and  the  body  will  fall. 

A  man  carrying  a  load  on  his  back  naturally 
bends  forward,  to  bring  his  line  of  direction  with- 
in the  base  formed  by  his  feet.  Otherwise,  the 
line  of  direction  falls  outside  of  the  base,  as 
shown  in  Fig.  63 ;  and  the  load,  if  heavy,  may 
pull  him  over  backward. 

159.   Of  different   bodies   of  the 
same  height,  that  which  has  the  broad- 
est base  is  the  hardest  to  overturn,  because  its  line  of  di- 
rection must  be  moved  the  farthest  to  fall  outside  of  its 

Fig.  64. 


Fig.  63. 


MYPTXAX  PYRAMIDS. 


is  the  Base  of  a  body  ?  When  a  body  is  supported  on  legs,  how  is  its  base  formed? 
158.  How  must  the  line  of  direction  fall,  for  a  body  to  stand  ?  Illustrate  this  with 
Figs.  60,  61,  62.  What  position  does  a  man  carrying  a  load  on  his  back  assume,  and 
why?  159.  Of  different  bodies  equally  high,  which  is  the  hardest  to  overturn? 


74  MECHANICS. 

base.  Hence  a  pyramid  is  the  most  stable  of  all  figures ; 
and,  of  different  pyramids  of  the  same  height,  that  which 
has  the  broadest  base  is  the  most  stable.  The  pyramids 
of  Egypt  have  withstood  the  storms  of  more  than  three 
thousand  years. 

The  stability  of  stone  walls  is  increased  by  making  them  broader  at  the 
base  than  at  the  top.  Candlesticks  and  inkstands  generally  spread  out  at  the 
bottom  that  they  may  not  be  easily  upset.  For  the  same  reason,  the  legs  of 
chairs  bend  outward  as  they  approach  the  floor.  A  three-legged  stool  or 
table  has  a  smaller  base  than  one  that  has  four  legs,  and  is  therefore  more 
easily  upset.  Hence,  also,  the  ease  with  which  a  man  standing  on  one  leg  is 
overturned. 

160.  A  ball  of  uniform  density  has  its  centre  of  gravity 
at  its  centre  of  magnitude.  When  such  a  ball  rests  on  a 
level  surface,  the  line  of  direction  falls  on  the  point  of  sup- 
port, and  it  therefore  remains  in  any  position  in  which  it  is 
placed.  But,  as  the  base  of  a  ball  consists  of  a  single  point, 
— the  point  in  which  it  touches  a  level  surface, — a  slight 
push  throws  the  line  of  direction  beyond  'the  base,  and 
causes  the  ball  to  roll. 

Fig.  65.  161.  When  a  ball  is  placed  on  a 

sloping  surface,  the  line  of  direction 
falls  outside  of  the  base,  and  the  ball 
begins  to  roll.  A  cube  placed  on  the 
same  sloping  surface  maintains  its  po- 
sition, because  the  line  of  direction 
falls  within  its  base.  See  Fig.  65,  in 
which  C  represents  the  centre  of  gravity. 

162.  Of  different  bodies  with  bases  equally  large,  the 
lowest  is  the  hardest  to  overturn,  because  its  line  of  direc- 
tion is  least  liable  to  fall  outside  of  its  base. 


Why  ?  What  is  the  most  stable  of  all  figures  ?  How  long  have  the  pyramids  of 
Egypt  stood  ?  Give  some  familiar  instances  in  which  the  base  of  a  body  is  made 
larger  than  the  top,  to  increase  its  stability.  Why  are  three-legged  chairs  and  tables 
easily  overturned  ?  160.  In  a  ball  of  uniform  density,  where  is  the  centre  of  gravity? 
What  is  said  of  the  stability  of  such  a  ball,when  resting  on  a  level  surface  ?  161.  When 
such  a  ball  is  placed  on  a  sloping  surface,  what  takes  place  ?  Compare  it,  in  this  re- 
spect, with  a  cube.  162.  Of  different  bodies  with  bases  equally  large,  which  is  the 


CENTRE   OP   GRAVITY. 


75 


Fig.  67. 


This  is  apparent  from  Figs.  66  and 

67.      The    unfinished    tower,   though 

leaning  far  over,  maintains  its  upright 
position,  the 
line  of  direc- 
tion falling 
within  the 

/.  "^/       base-     When 
made    higher 

by  the  addi- 
tion of  seve- 
ral stories,  as 

shown  in  Fig.  67,  it  will  fall,  because 

the  centre  of  gravity  has  been  raised, 

and  the  line  of  direction  now  falls  outside  of  the  base. 

High  chairs  for  children  are  unsafe,  unless  their  legs  spread  at  the  bot- 
tom. A  coach  with 

heavy  baggage  piled  °'     • 

on  its  top  is  in  danger 

of    upsetting    on    a 

rough  road.     On  the 

same  principle,  a  load 

of  stone  may  pass  safe- 
ly over  a  hill-side,  on 

which  a  load  of  hay 

would  be  overturned. 

Fig.  68  shows  that  the 

line  of  direction  in  the 

one    case  would  fall 

within  the  base,while 

in  the  other  it  would 

fall  outside  of  it. 

163.  The  lower  its  centre  of  gravity,  the  more  stable  a 
body  is.  Those,  therefore,  who  pack  goods  in  wagons  or 
vessels,  should  place  the  heaviest  lowest. 

This  principle  has  been  turned  to  account  in  building  leaning  towers.  The 
tower  of  Pisa,  which  is  the  most  remarkable  of  these  structures,  with  a  height 
of  150  feet,  leans  to  such  a  degree  that  its  top  overhangs  its  base  more  than 
12  feet ;  yet  it  has  stood  firm  for  centuries.  In  this  case,  the  centre  of  grav- 
ity has  been  brought  lower  than  it  would  otherwise  have  been,  by  the  use  of 
heavy  materials  at  the  bottom  and  lighter  ones  higher  up.  The  lower  stories 
are  of  dense  volcanic  rock,  the  middle  stories  of  brick,  and  the  upper  ones  of 


hardest  to  overturn  ?  Why  ?  Illustrate  this  point  with  Figs.  66  and  67.  Give  soma 
familiar  applications  of  this  principle.  163.  Why  do  those  who  pack  goods  in  wagons 
placo  the  heaviest  lowest  ?  In  what  has  this  principle  been  turned  to  account  ?  De- 


MECHANICS. 


Fig.  TO. 


an  exceedingly  porous  stone.    Thus  built,  the  tower  is  much  less  liable  te 
fall,  than  if  the  same  material  had  been  used  throughout. 

164.  When  the  centre  of  gravity  is  brought  beneath  the 
point  of  support,  the  stability  of  a  body 
is  still  further  increased. 

This  is  shown  in  Fig.  69.  To  balance  a  needle  on 
its  point  is  next  to  impossible,  on  account  of  the 
smallness  of  the  base,  and  the  height  of  the  centre  of 
gravity.  It  may  be  done,  however,  by  running  the 
head  of  the  needle  into  a  piece  of  cork,  C,  and  stick- 
ing into  opposite  sides  of  this  cork  two  forks,  A,  B,  at 
equal  angles.  The  whole  may  then  be  poised  upon 
the  needle's  point  on  the  bottom  of  a  wine-glass.  In 
this  case,  the  heavy  handles  of  the  forks  bring  the 
centre  of  gravity  below  the  point  of  support,  in  the 
stem  of  the  glass. 

The  common  toy  known  as  the  Rocking 
Horse,  represented  in  Fig.  70,  is  made  on  this 
principle.  To  a  horse  of  any  light  material, 
bearing  a  trooper  or  some  other  figure,  is  at- 
tached a  wire  to  which  a  ball  m#y  be  fastened. 
When  the  hind  legs  of  the  horse  are  placed  on 
the  stand  without  the  ball,  the  line  of  direction 
falls  outside  of  the  base,  and  the  horse  and  his 
rider  fall.  When  the  ball  is  attached,  how- 
ever, the  centre  of  gravity  is  brought  below 
the  point  of  support  ;  the  horse  will  then  main- 
tain its  upright  position,  and  by  moving  the 
ball  may  be  made  to  rock  up  and  down. 

165.  EFFECT  OF  ROTARY  MO- 
TION. —  Rotary  Motion,  that  is,  mo- 
tion round  an  axis,  may  keep  a  body  from  falling,  even 
when  its  line  of  direction  falls  outside  of  its  base.  Thus,  if 
a  boy  tries  to  balance  his  top  on  its  point,  he  finds  it  im- 
possible ;  but,  when  he  spins  it,  it  stands  as  long  as  the  ro- 
tary motion  continues.  The  centre  of  gravity  is  not  over 
the  point  of  support  all  the  time  the  top  is  spinning,  but  is 


ROCKING-HORSE. 


•cribe  the  tower  of  Pisa,  and  the  materials  of  which  it  is  built  164.  How  is  the  sta- 
bility of  a  body  further  increased  ?  Show  how  a  needle  may  be  balanced  on  its  point 
by  applying  this  principle.  Describe  the  Kocking  Horse,  and  explain  the  principle 
Involved.  165.  What  is  meant  by  Eotary  Motion  ?  What  is  one  of  its  effects  ?  Why 
does  a  top  fall  ever  when  we  try  to  balance  it  on  its  point,  but  not  fall  when  spinning  ? 


CENTRE   OF   GRAVITY  IN   MAN. 


constantly  moving  round  the  axis  of  motion;  and,  before 
the  top  can  fall  in  consequence  of  its  being  on  one  side  of 
the  axis,  it  reaches  the  other  side,  and  thus  counteracts  the 
previous  impulse.  Hence,  the  faster  the  top  revolves,  the 
steadier  it  is ;  as  its  motion  slackens,  it  gradually  reels  more 
and  more,  and  finally  falls. 

166.  CENTRE  OF  GRAVITY  IN  MAN. — The  centre  of 
gravity  in  the  body  of  a  man  lies  between  the  hips ;  the 
base  is  formed  by  lines  connecting  the  extremities  of  the 
feet.  A  man  enlarges  this  base,  and  therefore  stands  more 
firm,  when  he  turns  his  toes  out  and  places  his  feet  a  short 
distance  apart.  When  old  and  infirm,  he  enlarges  his  base 
and  increases  his  stability  still  further  by  using  a  cane. 

When  attempting  to  rise  from  a  sitting  position,  a  man 
must  either  bend  his  body  forward  or  draw  his  feet  back- 
ward, in  order  to  bring  his  centre  of  gravity  over  his  base ; 
otherwise,  he  will  fall  back  in  making  the  attempt.  So,  a 
person  who  keeps  his  heels  against  a  wall,  can  not  stoop 
without  falling,  because  he  has  no  room  to  throw  the  mid' 
die  of  his  body  far  enough  back  to  keep  the  line  of  direc' 
tion  within  the  base. 


Fig.  71. 


Nature  teach- 
es a  man  when  de- 
scending a  height 
to  lean  backward, 
and  when  ascend- 
ing to  lean  for- 
ward, as  shown  in 
Fig.  71.  In  like 
manner,  when 
carrying  a  weight 
on  one  side,  we 

sway  our  body  to  the  other,  like  the  man  with  the 
watering-pot,  in  Fig.  72.  We  find  it  easier  to 
carry  a  pail  of  water  in  each  hand  than  to  carry 
but  one,  because  the  weights  balance  each  other, 


Fig.  72. 


166.  Where  does  .the  centre  of  gravity  lie  in  a  man's  body  ?  How  may  a  man  increase 
his  stability  ?  When  attempting  to  rise  from  a  sitting  position,  what  must  a  man  do  ? 
Why  can  not  a  person  stoop,  if  he  keeps  his  heels  against  a  wall  ?  What  does  nature 
teach  a  man  to  do,  when  descending  a  height?  When  ascending  a  height?  "When 


MECHANICS. 


and  no  effort  is  necessary  to  keep  the  line  of  direction  within  the 
base. 

An  infant  that  has  not  learned  to  balance  itself  in  a  standing  position 
creeps  on  all  fours  without  danger,  because  it  thus  brings  its  centre  of  grav- 
ity lower  and  enlarges  its  base.  In  order  to  walk,  it  must  know  how  to  pre- 
serve its  balance ;  and,  as  some  practice  is  necessary  for  this,  the  child  in  its 
first  efforts  is  likely  to  fall.  The  same  is  the  case  with  a  dizzy  or  an  intoxi- 
cated person,  who  for  the  time  loses  the  power  of  preserving  his  balance — 
that  is,  of  keeping  his  line  of  direction  within  his  base. 

167.  "When  a  person  slips  on  one  side,  he  naturally  throws  out  his  arm  on 
the  other.  He  thus  seeks  to  bring  back  his  centre  of  gravity  over  his  base, 
and,  when  he  can  do  so,  he  saves  himself  from  falling.  A  person  skating  has 
to  use  his  arms  constantly  for  this  purpose.  Rope-dancers,  in  performing 

their  feats,  have  to  shift  their  centre 
of  gravity  from  point  to  point  with 
great  rapidity ;  and,  finding  their 
arms  insufficient  for  maintaining 
their  balance  on  the  rope,  they  use 
a  long  pole,  with  a  slight  motion  of 
which  they  can  throw  the  centre  of 
gravity  into  any  desired  position. 

168.  The  shepherds  of  Landes 
[lond],  in  the  south-west  of  France, 
have  turned  the  art  of  balancing  to 
good  account.  Having  to  tend  their 
sheep  in  a  region  covered  with  marsh 
in  winter  and  hot  sand  in  summer, 
they  mount  on  stilts  about  four  feet 
high.  Though  the  centre  of  gravity 
is  raised,  and  their  liability  to  fall 
thus  increased,  by  practising  from 
an  early  age  they  become  exceeding- 
ly expert  on  these  stilts,  and  can  not 
only  walk  on  them,  but  even  dance, 
and  run  so  fast  that  it  is  hard  for  a 
stranger  to  keep  up  with  them. 

169.  STABLE  AND  UNSTABLE  EQUILIBRIUM. — The  centre 
of  gravity  of  every  body  tends  to  get  to  the  lowest  possible 
point. 

carrying  a  weight  on  one  side  ?  Why  do  we  find  it  easier  to  carry  a  pail  of  water  in 
each  hand  than  to  carry  but  one  ?  Why  is  an  infant  safer  when  creeping  than  when 
attempting  to  walk ?  Why  does  an  intoxicated  person  reel?  167.  When  a  person 
slips  on  one  side,  what  does  he  do,  and  why?  How  do  rope-dancers  preserve  their 
balance  ?  168.  How  have  the  shepherds  of  Landes  turned  the  art  of  balancing  to 
practical  use  ?  169.  What  point  does  the  centre  of  gravity  tend  to  reach  ?  Illustrate 


8HEPHEBD8  OF  LAICDE8. 


STABLE   AND   UNSTABLE   EQUILIBRIUM. 


79 


A  ball  suspended  by  a  string,  as  in  Fig.  74,  and  re-  Fig.  74. 

leased  from  the  hand  at  K,  or  any  other  point,  will  not 
come  to  rest  till  it  reaches  L,  because  there  its  centre 
of  gravity,  B,  is  at  its  lowest  point.  Hence,  when  a 
pendulum  or  plummet  comes  to  rest,  it  always  hangs 
vertically. 

A  hammer,  no  matter  in  what  way  it  is  thrown  up, 
descends  with  its  iron  part  first,  because  the  centre  of 
gravity,  which  is  in  that  part,  tends  to  get  as  low  as 
possible.  For  the  same  reason,  a  shuttlecock  or  an 
arrow,  when  it  has  reached  its  highest  point,  turns 
and  descends  with  its  heaviest  part  foremost. 

170.  A  solid  body  resting  on  a  surface  in  such  a  way 
that  its  centre  of  gravity  is  lower  than  it  would  be  in  any 
other  position,  is  said  to  be  in  Stable  Equilibrium.  If  its 
centre  of  gravity  could  be  brought  lower  by  placing  it  dif- 
ferently, it  is  said  to  be  in  Unstable  Equilibrium. 


Fig.  75. 


Thus,  the  oval  body,  A  B,  represent- 
ed in  Fig.  75,  is  in  stable  equilibrium, 
because  its  centre  of  gravity,  C,  is  at 
its  lowest  possible  point ;  and  a  force 
applied  to  either  end  will  not  cause  it 
to  fall  over,  but  only  to  rock  to  and  fro. 


In  the  position  shown  in  Fig.  76,  it  is  in  unstable  equilibrium, 

because  its  centre  of  gravity  might  be  brought  lower ;  and  a 

slight  push  will  overturn  it  and  bring  it  to  the  position  shown  in  Fig.  75.     It 

is  hardly  possible  to  balance  an  egg  on  either  end ;  but  placed  on  its  side,  it 

rests  securely. 

171.  The  stability  of  a  sphere,  or  oval  body  like  an  egg, 
is  increased  by  cutting  it  into  two  equal  parts,  as  shown  in 
Fig.  77.  Bases  of  this  shape  are 
used  in  rocking  toys,  for  support- 
ing the  figures  of  men  and  animals. 
Of  this  shape,  also,  are  some  of  the 


Fig.  77. 


huge  Rocking  Stones  found  in  different  parts  of  Europe, 
which  are  so  nicely  poised  that  the  slightest  push  causes 
them  to  rock  to  and  fro,  while  a  dozen  men  can  not  over- 
turn them. 


this  with  Fig.  74.  When  a  pendulum  or  plummet  comes  to  rest,  how  does  it  hang? 
How  does  a  hammer,  a  shuttlecock,  or  an  arrow,  descend,  when  thrown  up  into  the 
air,  and  why  ?  170.  When  is  a  body  said  to  be  in  Stable  Equilibrium  ?  When,  in  Un- 
stable Equilibrium  ?  Apply  this  in  Figs.  75  and  76.  171.  How  may  the  stability  of  a 


80 


MECHANICS. 


Fig.  7& 


172.  PARADOXES.— The  tendency  of  the  centre  of  grav- 
ity to  reach  its  lowest  possible  point  sometimes  produces 
wonderful  -effects,  or  Paradoxes,  for  which  the  unlearned 
are  at  a  loss  to  account.  Thus,  we  know  that  a  ball  will  roll 
down  a  sloping  surface ;  but  a  ball  of  light  wood  may  be 
made  tor  oil  ^  up  a  sloping  surface  by  inserting  a  piece  of 
lead  in  one  side. 

The  ball  A,  for  instance,  loaded  on 
one  side  with  a  plug  of  lead  S,  is  placed 
on  a  sloping  surface.  The  centre  of 
gravity  C,  which  is  nearS,  at  once  tends 
to  reach  its  lowest  point ;  and  owing  to 
this  tendency  the  ball  rolls,  till  it  reaches 
the  position  shown  in  B. 

173.  In  like  manner,  a  double  cone,  or 
body  having  the  form  of  two  sugar-loaves  joined  at  their  large  ends,  may  be 
made  to  roll  up  an  inclined  plane.  Fig.  79  represents  two  rails,  joined  at  one 

end,  but  apart  and  somewhat  ele- 
vated at  the    other.      Place   the 
double  cone  at  the  middle  of  the 
rails  just  described,  and  instead 
of  rolling  down  to  the  narrow  end 
it  will  roll   up  to  the  wide  end. 
"*  This  is  because  the.  centre  of  grav- 
ity, though  apparently  going  up,  is  really  going  down  ;  for,  as  the  rails  di- 
verge, they  let  the  double  cone  further  down  between  them. 


.Fig.  79. 


sphere  or  oval  body  be  increased  ?  For  what  are  bases  of  this  shape  used  ?  What 
stones  are  of  this  shape  ?  172.  What  are  Paradoxes  ?  How  are  they  sometimes  pro- 
duced ?  How  may  a  ball  be  made  to  roll  up  a  sloping  surface  ?  Explain  the  principle 
involved,  with  Fig.  78.  173.  Describe  the  experiment  with  the  double  cone,  and  ex- 
plain the  principle. 


MOTIVE  POWERS.  /  81 


CHAPTER  V 

MECHANICS    (CONTINUED). 

THE  MOTIVE  POWER. THE   RESISTANCE. THE   MACHINE. 

STRENGTH    OF   MATERIALS. 

1 74.  IN  a  previous  chapter  we  have  treated  of  the  Laws 
of  Motion;  we  now  proceed  to  consider  the  following  prac- 
tical points : — 

I.  The  Motive  Power,  or  Force  by  which  motion  is  pro- 

duced. 

II.  The  Resistance  to  be  overcome,  or  work  to  be  done, 

which  is  always  opposed  to  the  Power. 

III.  The  Machine,  which  is  used  by  the  Power  in  over- 

coming the  Resistance,  when  it  does  not  itself  di- 
rectly act. 

IV.  The  Strength  of  the  Materials  employed. 

In  the  case  of  a  steamboat,  steam  is  the  Power  by  which  motion  is  pro- 
duced ;  the  weight  of  the  boat  is  the  Resistance,  which  constantly  opposes 
the  Power.  Since  steam  can  not  be  directly  applied  in  such  a  way  as  to  move 
the  boat,  a  Machine  is  used  to  aid  in  overcoming  the  Resistance ;  and  this 
Machine  is  the  engine.  On  the  strength  of  the  materials  employed  depend 
the  usefulness  and  safety  of  the  whole. 

Motive  Power*. 

175.  The  chief  powers  used  by  man  in  producing  mo- 
tion  are   gravity,  the   elastic  force   of  springs,   his   own 
strength,  the  strength  of  animals,  wind,  water,  and  steam. 

176.  Gravity. — Springs. — Gravity  is  applied  by  attach- 
ing weights  to  machinery,  which  they  keep  in  motion  by 
their  constant  downward  tendency,  as  in  certain  kinds  of 

174.  What  four  subjects  connected  with  Mechanics  are  treated  of  in  the  present 
ehaptcr  ?  In  the  case  of  a  steamboat,  what  is  the  power  ?  What,  the  resistance  ? 
What,  the  machine  ?  On  what  does  the  usefulness  of  the  whole  depend  ?  175.  Name 
the  chief  powers  employed  by  man  in  producing  motion.  176.  How  is  gravity  ap- 

4* 


82  MECHANICS, 

clocks.  When  the  weight  descends  so  far  that  it  reaches  a 
support,  the  machinery  ceases  to  move,  and  is  said  to  "  run 
down".  When  there  is  no  room  to  use  weights,  springs 
are  often  substituted  for  them,  as  in  the  works  of  watches. 
A  spring  is  made  of  steel,  or  some  other  elastic  substance ; 
which,  being  bent,  produces  motion  by  a  constant  effort  to 
unbend  itself. 

177.  Strength  of  Men  and  Animals. — With  his  own 
strength  man  can  produce  a  certain  degree  of  motion,  but 
not  such  as  accomplishes  the  grandest  results.  From  the 
strength  of  animals  he  derives  important  assistance.  Even 
rude  nations  tame  the  animals  around  them,  and  turn  their 
strength  to  account.  The  American  Indians,  when  first 
discovered,  had  not  learned  to  do  this ;  and  therefore,  like 
other  savages  who  rely  entirely  on  their  own  strength,  they 
had  made  no  great  advance  in  agriculture,  manufactures, 
or  any  other  branch  of  industry. 

The  horse  is  the  animal  whose  strength  is  most  widely 
and  advantageously  used.  For  continued  labor,  one  horse 
is  considered  equal  to  five  men.  A  horse  of  average  strength 
can  draw  a  load  of  a  ton,  on  a  good  road,  from  20  to  25 
miles  a  day. 

178.  Wind  and  Water. — Still  more  powerful  forces  are 
found  in  wind  and  water,  which  are  extensively  used  as 
moving  powers  by  all  civilized  nations. 

The  wind  is  brought  to  bear,  not  only  on  the  sails  of  vessels,  but  also  in 
mills  used  for  grinding  grain,  sawing  wood,  raising  water,  expressing  oil 
from  seeds,  &c.  Such  machines  are  called  "Wind-mills ;  they  were  introduced 
into  Europe  from  the  East,  about  the  time  of  the  Crusades.  The  great  objec- 
tion to  the  wind  as  a  moving  power,  is  its  irregularity,  for  in  still  weather  the 
machines  it  moves  are  useless. 

Water  is  a  very  powerful  and  useful  agent.    A  little  stream  is  often  a 

plied  ?  When  is  the  machinery  said  to  run  down  ?  When  there  is  no  room  to  use 
weights,  what  are  often  substituted  for  them  ?  How  does  a  spring  produce  motion  ? 
177.  What  is  said  of  the  strength  of  man  as  a  source  of  motion  ?  What,  of  the 
strength  of  animals  ?  What  animal  is  most  widely  used  ?  To  how  many  men  is  one 
horse  considered  equal  ?  As  regards  drawing,  what  is  a  day's  work  for  a  horse  of  av- 
erage strength  ?  178.  "What  sources  of  motion  are  still  more  powerful  ?  How  is  the 
wind  brought  to  bear  ?  What  are  machines  moved  by  the  wind  called  ?  Whence 
and  when  were  wind-mills  introduced  into  Europe  ?  What  is  the  great  objection  to 


STEAM,  AS   A   MOTIVE   POWEB.  83 

source  of  prosperity  and  wealth  to  an  extensive  region.  Affording  what  is 
called  "  water-power  ",  it  moves  huge  machines,  and  thus  affords  the  means 
of  manufacturing  easily  and  cheaply.  Water  was  first  used  as  a  motive  power 
by  the  Romans,  in  simple  machines  for  grinding  grain,  about  the  commence- 
ment of  the  Christian  era.  It  is  now  applied  in  various  kinds  of  machines, 
for  sawing,  spinning,  weaving,  grinding,  &c.  Though  a  stream  may  run  so 
high  in  spring  and  so  low  in  summer  as  to  be  useless  for  a  time,  there  is  far 
less  difficulty  from  these  causes  than  from  the  irregularity  of  the  wind. 

179.  Steam. — The  greatest  of  all  the  powers  employed 
by  man  is  STEAM,  or  the  vapor  generated  by  submitting 
water  to  a  high  degree  of  heat.  Steam  being  an  elastic 
fluid,  its  properties  and  applications  will  be  considered 
hereafter. 

180.  The  uses  of  steam  were  unknown  to  the  ancients ;  it  was  not  till  near 
the  close  of  the  seventeenth  century  that  its  importance  began  to  be  realized. 
Its  application  to  machinery  marks  an  era  in  the  world's  history,  and  has  in- 
vested man  with  immense  power  over  matter.  Driving  the  boat  and  car,  it 
bears  him  what  was  once  a  day's  journey  in  an  hour.  Applied  in  countless 
varieties  of  machines,  it  is  the  means  of  supplying  us  with  thousands  of  com- 
forts unknown  to  our  forefathers.  The  farmer  is  indebted  to  it  for  his  spade, 
toe,  rake,  scythe,  ploughshare,  and  all  his  implements.  It  helps  to  make 
the  shears  with  which  he  cuts  the  wool  from  his  sheep,  and  then  cards  the 
wool,  and  weaves  it  into  cloth.  It  separates  his  cotton  from  its  seed,  and 
turns  it  into  muslin  and  calico.  It  aids  the  builder  by  making  his  tools,  forg- 
ing his  nails  and  bolts,  moulding  his  ornaments,  polishing  his  marble,  cutting 
his  stone,  and  sawing  his  wood.  It  supplies  our  parlors  with  furniture,  our 
kitchens  with  cooking  utensils,  our  dining-rooms  with  glass  and  china,  knives 
and  forks.  It  knits,  twists,  washes,  irons,  dyes,  gilds,  grinds,  digs,  and 
prints ;  and  hardly  any  work  of  art  meets  our  eyes,  in  making  which  steam 
has  not  been  directly  or  indirectly  used.  It  does  all  this,  moreover,  with 
wonderful  precision  and  rapidity.  The  pyramids  of  Egypt,  we  are  informed, 
kept  100,000  men  at  work  twenty  years  in  their  erection.  It  has  been  com- 
puted that  one  powerful  steam-engine  would  have  done  as  much  work  in  the 
same  time  as  27,000  of  these  Egyptians. 

The  Resistance. 

181.  Whatever  opposes  the  Power  is  called  the  Resist- 
ance. 

the  wind  as  a  moving  power  ?  "What  is  said  of  water-power  ?  By  whom  and  when 
was  it  first  used  ?  For  what  purposes  is  it  now  employed  ?  What  are  the  disadvan- 
tages of  water  as  a  moving  power  ?  179.  What  is  the  greatest  of  the  powers  em- 
ployed by  man  ?  What  is  Steam  ?  180.  When  did  its  importance  begin  to  be  real- 
ized ?  What  has  been  the  result  of  its  application  to  machinery  ?  Enumerate  the 
different  articles  which  steam  is  constantly  employed  in  producing.  What  interest- 


84  MECHANICS. 

182.  The  resistance  is  not  always  of  the  same  character. 
It  may  be  a  weight  to  be  lifted,  as  a  pail  of  water  from  a 
well ;  or  a  body  to  be  moved  onward,  as  a  tram  of  cars ; 
or  a  wheel  to  be  turned,  as  in  a  mill ;  or  particles  to  be 
compressed,  as  in  packing  cotton  in  bales ;  or  cohesion  to 
be  overcome,  as  in  splitting  a  log  of  wood.     As  the  most 
usual  form  in  which  the  resistance  appears  is  that  of  a 
weight  to  be  moved,  the  term  Weight  is  often  used  instead 
of  Resistance,  with  reference  to  work  of  any  kind,  or  what- 
ever opposes  the  moving  power. 

183.  UNITS  OF  WORK. — The  efficiency  of  a  force  is  esti- 
mated by  the  resistance  it  can  overcome,  or  the  amount  of 
work  it  can  do.     In  order  to  compare  different  forces,  we 
must  have  a  uniform  unit  of  wor7f. 

The  unit  of  work  adopted  is  the  resistance  encountered 
in  raising  one  pound  through  the  space  of  a  foot.  Hence, 
to  raise  a  body  any  distance  constitutes  as  many  units  of 
work  as  there  are  pounds  in  the  body  multiplied  by  the 
number  of  feet  in  the  given  distance.  To  raise  2  pounds 
of  water  from  a  well  6  feet  deep,  is  equivalent  to  twice  6, 
or  12,  units  of  work.  To  lift  a  load  of  1,000  pounds  10 
feet  involves  10,000  units  of  work. 

184.  HORSE-POWERS. — In  estimating  large  amounts  of 
work,  it  is  customary  to  use  horse-powers  as  a  measure.     A 
horse  can  perform  33,000  units  of  work,  that  is,  can  raise 
33,000  pounds  a  foot,  in  a  minute.     An  engine,  therefore, 
that  can  perform  33,000  units  of  work  in  a  minute  is  said 
to  be  an  engine  of  one  horse-power ;  one  that  can  do  66,000 
units  of  work  in  a  minute  is  an  engine  of  2  horse-powers  ; 
and  so  on.     Hence  the  following 

Mule. — To  find  the  horse-power  of  an  engine,  divide  the 
number  of  pounds  it  is  capable  of  raising  one  foot  in  a  min- 
ute by  33,000. 

Jng  fact  is  stated  with  respect  to  the  pyramids  of  Egypt  ?  181.  What  is  the  Resist- 
ance ?  182.  Mention  some  of  the  different  forms  in  which  the  resistance  appears,  and 
give  examples.  What  term  is  often  used  instead  of  resistance,  and  why  ?  183.  How 
f»  the  efficiency  of  a  force  estimated  ?  To  compare  different  forces,  what  is  it  neces' 
eary  to  have  ?  What  is  the  unit  of  work  generally  adopted  ?  Give  examples. 


FRICTION.  85 

185.  FEICTION. — The  effect  of  the  moving  power  is  often 
diminished  by  Friction.  • 

Friction  is  the  resistance  which  a  moving  body  meets 
with  from  the  surface  on  which  it  moves. 

If  all  surfaces  were  perfectly  smooth,  there  would  be  no  friction  ;  but  even 
those  bodies  that  seem  the  smoothest  are  really  covered  with  minute  projec- 
tions and  depressions.  These  fit  into  each  other,  and  a  certain  degree  of 
force  is  required  to  raise  the  projections  of  the  one  surface  over  those  of  the 
other.  With  the  naked  eye  we  can  not  detect  any  unevenness  on  plate  glass 
or  polished  steel ;  yet,  if  we  view  either  through  a  microscope,  we  find  that 
its  surface  is  far  from  smooth,  and  hence  there  is  some  friction  even  when 
these  substances  are  rubbed  together. 

186.  Friction  opposes  motion  in  two  ways: — 

1.  By  increasing  the  resistance,  as  when  a  weight  is 
dragged  over  the  ground. 

2.  By  diminishing  the  force  before  it  is  applied  to  the 
resistance ;  as  in  machinery,  which  sometimes  loses  as  much 
as  one-third  of  its  power  by  the  rubbing  of  its  different  parts 
against  each  other. 

In  estimating  the  working  power  of  a  machine  for  practical  purposes,  it 
is  necessary  to  make  allowance  for  the  loss  occasioned  by  friction ;  but,  in 
merely  investigating  the  principles  of  Mechanics  and  the  construction  of  ma- 
chines, we  proceed  aa«if  the  surfaces  concerned  were  perfectly  smooth,  and 
no  such  thing  as  friction  existed. 

187.  Kinds  of  Friction. — There  are  two  kinds  of  fric- 
tion : — 

1.  Sliding  Friction,  produced  when  a  body  slides  on  a 

surface,  like  the  runners  of  a  sleigh. 

2.  Rolling  Friction,  produced  when  a  body  rolls  on  a 

surface,  like  the  wheels  of  a  wagon. 

188.  Between  any  given  surface  and  moving  body,  slid- 
ing friction  is  much  greater  than  rolling  friction.     Hence 
we  roll  a  barrel  of  flour  over  the  ground  instead  of  drag- 

184.  How  are  large  amounts  of  work  estimated  ?  What  is  meant  by  a  Tiorse-power  t 
Give  an  example.  How  may  the  horse-power  of  an  engine  be  found?  185.  By  what 
is  the  effect  of  the  moving  power  often  diminished  ?  What  is  Friction  ?  How  is 
it  that  friction  is  exhibited  even  between  surfaces  that  appear  smooth  ?  Give  an  ex- 
ample. 186.  In  how  many  ways  does  friction  oppose  motion  ?  Mention  them.  When 
te  it  necessary  to  make  allowance  for  friction,  and  when  not  ?  187.  How  many  kinds 
of  friction  are  there  ?  Name  them,  and  tell  how  each  is  produced.  188.  Between  any 


86 


MECHANICS. 


Fig.  80. 


ging  it,  and  place  a  weight  that  is  to  be  moved  in  a  cart,  or 
suspend  it  between  wheels,  instead  of  harnessing  a  horse 
directly  to  it. 

On  the  same  principle,  we  place  rollers  under  a  block  of  marble,  and  fasten 
castors,  or  small  wheels,  to  the  legs  of  heavy  pieces  of  furniture.    Rollers  are 

also  used  with  advantage  in  pushing 
a  ponderous  packing-box  up  an  in- 
clined plane  into  a  cart,  as  shown  in 
Fig.  80.  In  all  these  cases,  sliding 
friction  is  converted  into  rolling,  and 
the  resistance  is  thus  diminished.  The 
larger  the  wheels  and  rollers  employ- 
ed, up  to  a  certain  limit,  the  greater 
the  gain ;  but  even  small  ones  mate- 
rially lessen  the  friction. 

Rolling  friction,  on  the  other  hand, 
may  be  converted  into  sliding.  This  is  done  when  the  wheels  of  a  heavily 
loaded  stage  or  wagon  descending  a  steep  hill  are  locked,  that  is,  prevented 
from  turning  by  an  apparatus  provided  for  the  purpose.  The  resistance  is 
thus  increased  to  such  a  degree  that  the  load  can  descend  in  safety.  On  the 
same  principle,  brakes  are  applied  to  the  wheels  of  cars,  to  stop  them  the 
sooner. 

189.  Laws  of  Friction. — Several  important  laws  relating  to  friction  have 
been  settled  by  experiments.    In  making  these,  the  apparatus  represented  in 

Fig.  81  has  been  gsed.  D  E  is  a  table, 
on  which  rests  tUe  block  C.  A  string, 
passing  over  the  pulley  B,  connects  this 
block  with  a  scale,  A.  By  putting 
weights  in  the  scale  till  the  block  moves, 
we  are  enabled  to  measure  its  friction ; 
and,  by  making  the  block  of  different 
materials,  varying  its  size  and  surface, 
and  allowing  it  to  remain  a  longer  or 
shorter  time  on  the  table,  the  following 
laws  have  been  established  : — 

1.  The  friction  of  a  body  is  greater  when  it  commences 
moving  than  after  it  has  been  moving  for  a  time.     Thus  it 


(Tlven  surface  and  moving  body,  how  does  sliding  friction  compare  with  rolling  fric- 
tion ?  Mention  some  familiar  cases  in  which  we  convert  sliding  into  rolling  friction, 
to  lessen  the  resistance.  What  is  said  of  the  size  of  the  wheels  and  rollers  employed  ? 
In  what  cases  is  rolling  friction  converted  into  sliding  ?  189.  How  have  the  facts  re- 
lating to  friction  been  settled  ?  Describe  the  apparatus  employed  for  this  purpose. 
When  is  the  friction  of  a  body  greatest  ?  Between  what  bodies  and  surfaces  is  fric- 


LAWS   OF   FRICTION.  87 

takes  a  heavier  weight  to  start  the  block  C  than  it  does  af- 
terwards to  keep  it  in  motion. 

2.  Friction  is  greater  between  soft  bodies  than  hard 
bodies,  and  between  rough  surfaces  than  smooth  ones.     A 
sled  that  can  hardly  be  moved  over  a  newly  ploughed  field, 
is  drawn  without  difficulty  over  a  frozen  pond. 

3.  In  many  cases,  friction  is  increased  by  letting  the 
surfaces  remain  in   contact.     At  the   end  of  five  or  six 
days,  it  has  been  found  to  be  fourteen  times  as  great  as  at 
first. 

4.  Between  the  same  surfaces,  friction  is  proportioned 
to  the  weight  of  the  moving  body.     The  friction  of  a  block 
weighing  20  pounds  is  twice  as  great  as  that  of  a  ten-pound 
block. 

5.  "Within  certain  limits,  friction  is  not  increased  by  ex- 
tent of  plane  surface.    As  long  as  the  weight  of  a  body  re- 
mains the  same,  its  friction  will  not  vary,  whether  'it  rests 
on  a  larger  or  smaller  base.    In  Fig.  81,  the  block  C  has  its 
upper  side  hollowed  out,  so  that,  if  turned  over,  it  will  rest 
merely  on  two  ridges ;  yet  the  friction  will  be  the  same  when 
it  rests  on  that  side  as  on  the  other. 

190.  Modes  of  Lessening  Friction. — No  means  has  yet 
been  found  of  doing  away  with  friction  altogether ;  but  it 
may  be  lessened  in  three  ways : — 

1.  By  smoothing  and  polishing  the  surfaces. 

2.  By  putting  grease  or  some  other  lubricant,  as  it  is 
called,  between  the  surfaces.    This  fills  up  their  depressions. 
Finely  powdered  plumbago   (the  common  black-lead  used 
in  pencils),  dry  for  wooden  surfaces  and  mixed  with  grease 
for  metallic  ones,  is  one  of  the  best  articles  used  for  this 
purpose.    The  wood-sawyer  greases  his  saw  to  make  it 
move  easily,  and  cartmen  and  carriage-drivers  keep  the 


tion  greatest  ?  In  many  cases,  how  may  friction  be  increased  ?  Between  the  same 
surfaces,  to  what  is  friction  proportioned  ?  What  effect  is  produced  on  the  friction  of 
a  body  by  increasing  its  surface  ?  Exemplify  this  with  the  figure.  190.  Can  friction 
be  entirely  removed  ?  In  how  many  ways  may  it  be  lessened  ?  What  is  the  first  of 
these  ?  What,  the  second  ?  What  article  makes  one  of  the  best  lubricants  ?  By 
whom  are  lubricants  used  ?  How  may  the  friction  of  a  wheel  be  diminished  ?  What 


88  MECHANICS. 

axles  of  their  wheels  well  covered 
with  some  lubricating  preparation. 

3.  The  friction  of  a  wheel  may 
be  diminished  by  making  its  axle, 
that  is,  the  cylinder  running  through 
the  centre,  turn  on  the  circumfer- 
ences of  two  other  wheels  at  each 
I  end,  as  shown  in  Fig.  82.  Such 

FRICTION  WHEELS.  wheels  are  called  Friction  Wheels. 

They  are  used  in  delicate  machinery. 

191.  Uses  of  Friction.— Though  friction  occasions  a  great  loss  of  power, 
It  is  not  without  its  beneficial  effects.  A  river  is  prevented  from  rushing 
madly  through  its  channel  by  the  friction  of  its  waters  on  its  banks  and  bed. 
A  tempest  gradually  loses  its  force  by  the  friction  of  the  air  against  the  pro- 
jections on  the  earth's  surface.  It  is  friction  that  prevents  the  fibres  of  wool, 
hemp,  and  cotton,  when  twisted  together,  from  slipping  on  each  other  and 
giving  way.  Without  friction  nails  would  be  useless,  for  they  would  draw 
right  out ;  the  wheels  of  a  carriage  would  turn  on  the  ground  without  moving 
it  forward ;  and  neither  man  nor  beast  could  walk.  It  is  the  friction  of  our 
feet  on  the  ground  that  enables  us  to  take  steps  :  when  the  fruition  is  lessened, 
as  on  smooth  ice,  we  walk  with  difficulty ;  were  there  no  friction,  we  should 
find  it  impossible  to  walk  at  all. 

Machines. 

192.  Machines  are  instruments  used  to  aid  the  Power 
in  overcoming  the  Resistance. 

1 93.  Simple  machines  used  by  the  hand,  are  called  Tools ; 
as,  the  chisel,  the  saw. 

1 94.  Machines  of  great  power  are  called  Engines ;  as, 
the  steam-engine,  the  fire-engine. 

195.  Machines  merely  aid  the  power  in  its  action  ;  they 
can  not  create  power.    This  follows  from  the  inertia  of  mat- 
ter.    The  mightiest  engine,  therefore,  remains  at  rest  until 
acted  on  by  some  motive  power  ;  and,  when  thus  acted  on, 
it  can  not  increase  the  power  in  the  smallest  degree,  but  on 


nre  such  Vheels  called  ?  191.  Mention  some  of  the  beneficial  effects  of  friction. 
192.  What  are  Machines  ?  193.  What  are  Tools  ?  194.  What  are  Engines  ?  195.  What 
do  machines  merely  do?  Why  can  not  a  machine  increase  the  power?  Illustrate 
this  principle  in  the  case  of  a  man  who  can  raise  100  pounds  of  coal  a  minute  from  a 


PERPETUAL  MOTION.  89 

the  other  hand  diminishes  it,  more  or  less  according  to  the 
friction  of  its  parts. 

If  a  man  standing  over  a  pit  100  feet  deep  can,  in  the  space  of  a  minute, 
just  pull  to  the  top  a  tub  containing  100  pounds  of  coal,  no  machine  can  ena- 
ble him  to  raise  a  single  pound  more  in  the  same  time.  By  using  pulleys,  he 
may,  to  be  sure,  raise  600, 800,  or  1,000  pounds  at  a  time,  but  it  will  take  him 
6,  8,  or  10  times  as  long  as  before ;  and,  therefore,  in  the  same  time  he  will  do 
no  more  work  than  with  his  hands  alone — but  less,  on  account  of  the  friction 
of  the  pulleys.  So,  a  certain  amount  of  steam,  just  capable  of  performing 
•50,000  units  of  work  in  a  minute,  can  not  by  any  machinery  be  made  to  per- 
form a  single  additional  unit  of  work  in  the  same  time.  Hence  the  great  uni- 
versal law  which  follows  : — 

196.  W7iat  a  machine  gains  in  amount  of  work,  it  loses 
in  time;  and  what  it  gains  in  time,  it  loses  in  amount  of 
icork. 

Let  us  apply  this  law.  A  quantity  of  steam  capable  of  moving  50,000 
pounds  a  foot  in  a  second,  may  be  made  to  move  100,000  pounds  a  foot,  but 
it  will  be  two  seconds  in  doing  it ;  or  it  may  move  the  weight  a  foot  in  half  a 
second,  but  in  that  case  it  will  move  no  more  than  25,000  pounds.  Under  no 
circumstances  can  there  be  a  gain  in  units  of  work  without  a  corresponding 
loss  of  time,  or  a  gain  in  time  without  a  corresponding  loss  of  units  of  work. 

197.  PERPETUAL    MOTION. — By  Perpetual   Motion  is 
meant  the  motion  of  a  machine,  which,  without  the  aid  of 
any  external  force,  on  once  being  set  in  operation,  would 
continue  to  move  forever,  or  until  it  wore  out. 

Such  a  machine  many  have  tried  to  invent,  but  without 
success.  Friction  and  the  resistance  of  the  air  are  con- 
stantly opposing  the  action  of  machinery ;  and  as  matter, 
on  account  of  its  inertia,  can  generate  no  power  that  will 
compensate  for  this  loss,  every  machine  must  in  time  come 
to  rest,  unless  some  external  force,  such  as  wind,  water,  or 
steam,  keeps  acting  upon  it.  Hence  Perpetual  Motion  is 
impossible. 

198.  ADVANTAGES  OF  USING  MACHINERY. — If  no  addi- 
tional power  is  generated  by  machinery,  but  there  is  an 
actual  loss  from  the  friction  of  its  parts,  why  is  it  employed  ? 
— Because  in  other  respects  its  use  is  attended  with  impor- 
tant advantages,  among  which  are  the  following  : — 

pit  100  feet  deep.    Give  another  illustration.    196.  What  is  the  great  universal  law 
of  machines?    Apply  this  law  practically.    19T.  What  is  meant  by  Perpetual  Mo- 


90 


MECHANICS. 


Fig.  83. 


1.  Machinery  enables  us,  with  a  certain  amount  of  pow- 
er, by  taking  a  longer  time,  to  do  pieces  of  work  that  we 
could  not  otherwise  do  at  all. 

Thus,  a  farmer  with  a  crow-bar,  as 
shown  in  Fig.  83,  can  move  a  rock  which 
with  his  hands  alone  he  could  not  stir. 
With  the  aid  of  two  other  men,  he  could 
carry  it  or  push  it  where  he  wanted,  in 
one- third  of  the  time  that  he  could  move 
it  there  alone  with  the  crow-bar ;  but  he 
may  not  have  two  others  at  hand  to  help 
him. 

"With  machinery  10  men  may  do  the 
work  of  1,000.  Of  course  it  will  take 
them  100  times  as  long ;  but  this  loss  of  time  is  of  little  consequence,  com- 
pared with  the  difficulty  of  getting  a  thousand  men  together  and  placing  them 
so  as  to  work  without  interfering  with  each  other.  Some  heavy  pieces  of 
work  are  of  such  a  nature  that  but  few  laborers  can  get  around  them  at  a 
time ;  in  these  cases,  unless  the  work  can  be  divided,  which  is  not  always 
possible,  it  must  remain  undone  without  the  aid  of  machinery. 

2.  Machinery  enables  us  to  use  our  power  more  con- 
veniently. 

The  farmer  removes  a  rock  from  his  field  with  less  difficulty  and  fatigue 
by  means  of  a  crow-bar  than  if  he  stooped  over  to  lift  it  with  his  hands.  The 
porter  with  his  block  and  tackle  hoists  a  box  of  goods  to  a  loft  with  far  greater 
ease  than  he  could  push  or  carry  it  up.  The  apparatus  he  uses  enables  him 
to  hoist  the  load  by  pulling  down  upon  a  rope,  and  when  pulling  down  his 
weight  aids  his  strength. 

3.  Machinery  enables  us  to 
use  other  motive  powers  besides 
our  own  strength. 

A  horse  without  machinery  can  not  lift 
a  weight ;  but  he  does  it  readily  with  the 
aid  of  the  simple  apparatus  shown  in  Fig. 
84.  Steam,  applied  directly  to  a  boat, 
can  not  move  it  forward ;  it  is  only  with 
the  help  of  machinery  that  it  causes  the 
wheel  to  revolve  and  thus  produces  mo- 
tion. Here,  as  in  all  other  cases,  the 

tion?  Show  that  perpetual  motion  is  impossible.  198.  If  no  additional  power  is  gen- 
erated by  machinery,  why  is  it  used  ?  What  is  the  first  advantage  of  using  machine- 
ry ?  Give  an  example.  If,  with  machinery,  10  men  can  do  the  work  of  1,000,  how 
Vong  comparatively  will  it  take  them  ?  In  some  pieces  of  work,  what  difficulty  pre- 


STRENGTH    OF   MATERIALS.  91 

power  is  not  created  by  the  machinery,  but  merely  transmitted  in  a  way 
to  make  it  effective. 

Strength  of  Materials. 

199.  There  is  a  limit  to  the  power  of  all  machinery; 
and  this  limit  is  the  strength  of  the  materials  of  which  it  is 
made.     Machines  that  work  well  in  small  models  sometimes 
utterly  fail  when  made  of  full  size,  because,  when  the  resist- 
ance is  increased  and  their  own  weight  is  added,  no  mate- 
rial can  be  found  strong  enough  to  stand  the  strain. 

Nature,  also,  recognizes  this  limit  of  size.  Animals,  after  attaining  a  cer- 
tain age,  cease  to  grow.  If  they  kept  on  growing,  they  would  soon  reach 
such  a  size  and  weight  that  they  could  not  move.  If  there  were  an  animal 
much  larger  than  the  elephant,  it  would  stagger  under  its  own  weight,  unless 
its  bones  and  muscles  were  thicker  and  firmer  than  any  with  which  we  are 
now  acquainted.  Fish,  on  the  contrary,  being  supported  by  the  water,  move 
freely,  no  matter  how  heavy  they  may  be.  Whales  have  been  found  over  50 
feet  long  and  weighing  70  tons — a  monstrous  size  and  weight,  which  no  land 
animal  could  support. 

200.  To  determine  how  great  a  strain  given  materials 
will  bear,  and  how  they  may  be  put  together  with  the 
greatest  advantage,  becomes  an  important  question  in  Prac- 
tical Mechanics.     The  relative  strength  of  different  sub- 
stances has  been  treated  of  under  the  head  of  Tenacity,  on 
page  23.     The  following  general  principles  relating  to  rods, 
beams,  <fcc.,  should  be  remembered. 

1.  Rods  and  beams  of  the  same  material  and  uniform 
size  throughout,  resist  forces  tending  to  break  them  in  the 
direction  of  their  length,  with  different  degrees  of  strength, 
according  to  the  areas  of  their  ends. 

Let  there  be  two  rods  of  equal  length ;  if  the  areas  of  their  ends  are  re- 
spectively 6  and  3  square  inches,  the  one  will  bear  twice  as  great  a  weight 

Bents  itself?  What  is  the  second  advantage  of  using  machinery  ?  How  is  this  exem- 
plified in  the  case  of  the  farmer  ?  How,  in  the  case  of  the  porter?  What  is  the  third 
advantage  gained  by  using  machinery  ?  Illustrate  this  in  the  case  of  a  horse.  In  the 
case  of  steam.  In  both  of  these  cases,  what  does  the  machinery  merely  do  ?  199.  What 
limit  is  there  to  the  power  of  all  machinery  ?  Why  do  machines  often  fail,  though 
small  models  of  them  work  well  ?  Show  how  nature  recognizes  a  limit  of  size.  How 
is  it  that  fish  can  move,  though  much  larger  and  heavier  than  land  animals  ?  200.  What 
important  question  is  presented  in  Practical  Mechanics  ?  What  is  the  first  principle 
laid  down  respecting  rods  and  beams  ?  Give  an  example.  When  a  rod  is  very  long, 


92  MECHANICS. 

without  breaking  as  the  other.  This  law  applies,  no  matter  what  the  shape 
of  the  rods  may  be. 

2.  When  a  very  long  rod  is  suspended  vertically,  its 
upper  part,  having  to  support  more  of  the  weight  of  the 
rod  than  any  other,  is  the  most  liable  to  break. 

3.  The  strength  of  a  horizontal  beam  supported  at  each 
end  diminishes  as  the  square  of  its  length  increases. 

If  two  beams  thus  placed  are  respectively  6  feet  and  3  feet  long,  the 
strength  of  the  shorter  will  be  to  that  of  the  longer  as  the  square  of  6  to  tho 
square  of  3, — that  is,  as  36  to  9,  or  4  to  1. 

4.  A  horizontal  beam  supported  at  each  end,  is  most 
easily  broken  by  pressure  or  a  suspended  weight  in  the 
middle,   and  increases  in   strength  as  either  end  is   ap- 
proached.    If,  therefore,  a  beam  of  uniform  strength  is  re- 
quired, it  should  gradually  taper  from  the  middle  towards 
the  ends. 

5.  A  given  quantity  of  material  has  more  strength  when 
disposed  in  the  form  of  a  hollow  cylinder  than  in  any  other 
form  that  can  be  given  it.     Nature  constantly  uses  hollow 
cylinders  in  the  animal  creation,  as  in  bones  and  the  tubes 
of  feathers;  and  the  artisan,  imitating  nature,  employs  it 
in  many  cases  where  strength  and  lightness  are  to  be  com- 
bined. 

EXAMPLES  JFOR   PRACTICE. 

1.  (See  §§  183, 184.)    What  is  the  horse-power  of  a  steam-engine  that  can  do 

1,650,000  units  of  work  in  a  minute? 

2.  What  is  the  horse-power  of  an  engine  that  can  raise  2,376  pounds  1,000 

feet  in  a  minute  ? 

3.  What  is  the  horse-power  of  an  engine  that  can  raise  1,000  pounds  2,376 

feet  in  a  minute  ? 

4.  A  fire-engine  can  throw  220  pounds  of  water  to  a  height  of  75  feet  every 

minute ;  what  is  its  horse-power  ? 

5.  A  cubic  foot  of  water  weighs  621/.,  pounds.    How  many  horse-powers  are 

required  to  raise  200  cubic  feet  of  water  every  minute  from  a  mine  132 
feet  deep  ? 

what  part  of  it  is  most  likely  to  break  ?  What  law  is  given  respecting  the  strength  of 
a  horizontal  beam  supported  at  each  end  ?  Give  an  example.  In  what  part  is  a  hor- 
izontal beam  supported  at  each  end  most  easily  broken  by  pressure  ?  What  shape 
gives  a  beam  uniform  strength  ?  In  what  form  must  a  given  quantity  of  material  be 
disposed,  to  have  the  most  strength  ? 


EXAMPLES  FOR  PRACTICE.  93 

6.  A  locomotive  draws  a  train  of  cars,  the  resistance  of  which  (caused  by 

friction,  &c.)  is  equivalent  to  raising  1,000  pounds,  15  miles  an  hour; 
what  is  its  horse-power  ? 

[Find  how  many  feet  the  locomotive  draws  the  train  in  a  minute,  and  then 
proceed  as  before.} 

7.  How  many  pounds  can  an  engine  of  10  horse-powers  raise  in  an  hour  from 

a  mine  100  feet  deep  ? 

8.  A  certain  man  has  strength  equivalent  to  l/8  of  one  horse-power ;  how 

many  pounds  can  he  draw  up  in  a  minute  from  a  pit  25  feet  deep  ? 

9.  (See  §  189,  Fourth  Law.)  If  the  friction  of  a  train  of  cars  weighing  50  tons, 

on  a  level  railroad,  be  equivalent  to  a  weight  of  500  pounds,  what  will  be 
the  friction  of  a  train  weighing  25  tons  ?  of  one  weighing  100  tons  ?  of 
one  weighing  60  tons  ? 

10.  (See  §§  195, 196.)   G  can  just  draw  75  pounds  of  coal  a  minute  out  of  a 
mine.    With  the  aid  of  a  system  of  pulleys,  he  can  raise  225  pounds  at  a 
time;  the  friction  being  equivalent  to  75  pounds,  how  many  minutes 
will  he  be  in  raising  the  load  ? 

[In  practical  questions  of  this  kind,  the  friction  must  be  added  to  tJie  resist- 
ance.} 

11.  With  a  certain  machine,  one  man  can  do  as  much  as  eight  men  without 
the  machine.    Allowing  the  friction  of  the  machine  to  be  equal  to  one- 
fourth  of  the  resistance,  how  much  longer  will  he  be  in  doing  a  certain 
amount  of  work  than  they  ? 

12.  (See  §  200.)     [The  area  of  a  rectangular  surf  ace  is  found  by  multiplying 
its  length  by  its  breadth  ;  that  of  a  triangle,  by  multiplying  half  its  base 
by  its  perpendicular  height.']    Which  will  support  the  greater  weight 
without  breaking,  a  joist  whose  section  is  4  inches  long  by  5  broad,  or  one 
of  the  same  kind  of  wood,  3  inches  by  8  ? 

13.  Which,  when  suspended,  will  bear  the  greater  weight  without  breaking, 
a  square  rod  of  iron  whose  end  is  3  inches  by  3,  or  a  rod  whose  cross  sec- 
tion is  a  triangle  with  a  base  of  6  inches  and  a  perpendicular  height  of  2  ? 

14.  Two  rods  of  copper,  of  equal  length  and  uniform  thickness,  have  ends  re- 
spectively 4  inches  by  2,  and  17  inches  by  half  an  inch.    Which,  when 
suspended,  will  support  the  greater  weight  ? 

15.  Two  horizontal  beams  of  the  same  material,  breadth,  and  thickness,  sup- 
ported at  both  ends,  are  respectively  2  and  14  feet  long.    Which  is  *he 
stronger  of  the  two,  and  how  many  times  ? 


94  MECHANICS. 


CHAPTER  VIII. 

MECHANICS    (CONTINUED). 

THE   MECHANICAL   POWERS. 

NUMEROUS  and  varied  as  machines  are,  they  are  all 
combinations  of  six  Simple  Mechanical  Powers,  known  as 
the  Le'-ver,  the  Wheel  and  Axle,  the  Pulley,  the  Inclined 
Plane,  the  Wedge,  and  the  Screw.  •  These  we  shall  con- 
sider in  turn. 

The  l<ever. 

201.  A  Lever  is  an  inflexible  bar,  capable  of  being  moved 
about  a  fixed  point,  called  the  Fulcrum. 

The  lever  is  the  simplest  of  the  mechanical  powers.  Its  properties  were 
known  as  far  back  as  the  time  of  Aristotle,  350  years  B.  c.  Archimedes,  a 
hundred  years  later,  was  the  first  to  explain  them  fully. 

202.  KINDS  OF  LEVER. — In  the  lever  three  things  are  to 
be  considered ;  the  fulcrum,  or  point  of  support,  the  weight, 
and  the  power.     Two  of  these  are  at  the  ends  of  the  bar, 
while  the  other  is  at  some  point  between  them.    According 
to  their  relative  position,  we  have  three  kinds  of  levers : — 

Fig.  85.  A  Lever  of  the  First  Kind  is 

'&       [      one  in   which  the  fulcrum  is  be- 
I      tween  the  power  and  the  weight ; 
^p  as  in  Fig.  85,  where  F  represents 
the  fulcrum,  P  the  power,  and  W 
the  weight. 

A  Lever  of  the  Second  Kind  is  one  in  which  the  weight 
is  between  the  power  and  the  fulcrum  ;  as  in  Fig.  86. 

Of  what  are  all  machines  combinations  ?  Name  the  six  Simple  Mechanical  Pow- 
ers. 201.  What  is  a  Lever  ?  How  does  the  lever  compare  with  the  other  mechan- 
ical powers?  How  long  ago  was  it  known  ?  202.  In  the  lever,  how  many  things  are 
to  be  considered  ?  According  to  their  relative  position,  how  many  kinds  of  levers 
are  there  ?  What  is  a  Lever  of  the  First  Kind  ?  What  is  a  Lever  of  the  Second  Kind  ? 


95 


TUB  CKOW-BAB. 


A  Lever  of  the  Third  Kind  is  one  in  which  the  power  is 
between  the  weight  and  the  fulcrum  ;  as  in  Fig.  87. 

203.  LEVEES  OF  THE  FIRST  KIND. — In  levers  of  the  first 
kind,  the  relative  position  of  the  three  important  points  is 

POWER  FULCRUM  WEIGHT   OR   WEIGHT  FULCRUM  POWER. 

Fig.  88  shows  one  of  the  cornmon-  Fig.  §8. 

est  forms  in  which  this  kind  of  lever  ap- 
pears,— the  crow-bar.  The  power  is 
applied  at  the  handle.  The  weight  is  at 
the  other  end,  and  consists  of  something 
to  be  moved.  The  fulcrum  is  a  stone  on 
which  the  crow-bar  rests.  Using  an  in- 
strument in  this  way  is  called  prying. 

204.  The  nearer  the  fulcrum  is  to  the  weight  the  greater 
the  advantage  gained,  and  consequently  the  greater  the 
space  that  P  will  have  to  pass  through  in  moving  W  a  given 
distance.     This  principle  is  stated  in  the  following 

Law. —  With  levers  of  the  first  kind,  intensity  of  force 
is  gained,  and  time  is  lost,  in  proportion  as  the  distance 
between  the  power  and  the  fulcrum  exceeds  the  distance  be- 
tween the  weight  and  the  fulcrum. 

Thus,  in  Fig.  88,  if  the  distance  from  P  to  F  be  five  times  as  great  as  that 
from  W  to  F,  a  pressure  of  10  pounds  at  P  will  just  counterbalance  a  weight 
of  50  pounds  at  W,  and  will  therefore  move  anything  under  50  pounds ; 
while,  for  every  inch  that  "W  is  moved  upward,  P  will  have  to  move  five 
inches  downward. 

The  distance  through  which  the  power  must  pass,  to  move  a  weight  vast- 
ly greater  than  itself,  becomes  an  important  matter  in  practical  applications 
of  the  lever.  When  Archimedes  saw  the  immense  power  that  could  be  ex- 


What  is  a  Lever  of  the  Third  Kind  ?  203.  In  levers  of  the  first  kind,  what  is  the  rel- 
ative position  of  the  three  important  points  ?  Give  a  familiar  example  of  a  lever  of 
the  first  kind,  and  show  its  operation.  204  What  is  the  law  of  levers  of  the  first 
kind?  Illustrate  this  with  Fig.  88.  What  is  sometimes  an  important  matter  in  prac- 


96 


MECHANICS. 


Fig.  90. 


erted  with  this  instrument,  he  declared  that  with  a  place  to  stand  on  he  could 
move  the  earth  itself.  He  did  not  say  how  far  he  would  have  to  travel  to  do 
this,  in  consequence  of  the  great  disproportion  between  his  strength  and  the 
earth's  bulk.  Allowing  that  he  had  a  place  to  stand  on  and  a  lever  strong 
enough,  and  could  pull  its  long  arm  with  a  force  of  30  pounds  through  two 
miles  every  hour,  it  would  have  taken  him,  working  ten  hours  a  day,  over 
one  hundred  thousand  millions  of  years  to  move  the  earth  a  single  inch ! 

205.  The  Balance. — When  bodies  of  equal  weight  are 

supported  by  the  arms  of  a  lever,  they  will  balance  each 

Fig.  89.  other  when  placed  at  equal  distances 

% ^ %  from  the  fulcrum,  as  in  Fig.  89.  They 

are  then  said  to  be  in  equilibrium. 

On  this  principle  the  com- 
mon Balance,  represented  in 
Fig.  90,  is  constructed.  A 
beam  is  poised  on  the  top  of 
a  pillar,  so  as  to  be  exactly 
horizontal.  From  each  end 
of  the  beam,  at  equal  dis- 
tances from  the  fulcrum,  a 
pan  is  suspended  by  means 
of  cords.  The  object  to  be 
weighed  is  placed  in  one  of 
these  pans,  and  the  weights 
in  the  other. 

When  great  accuracy  is 
required,  the  beam  is  bal- 
anced on  a  steel  knife-edge ; 
the  friction  being  thus  les- 
sened, it  turns  more  easily.  A  balance  capable  of  weighing  ten  pounds  has 
been  made  so  sensitive  as  to  turn  with  the  thousandth  part  of  a  grain. 

206.  The  balance  weighs  correctly  only  when  the  arms  of  the  beam  are 
exactly  equal.  Hence  dishonest  tradesmen  sometimes  defraud  those  with 
whom  they  deal  by  throwing  the  fulcrum  a  little  nearer  one  end  of  the  beam 
than  the  other.  When  buying,  they  place  the  commodity  to  be  weighed  in 
the  scale  attached  to  the  short  arm ;  and,  when  selling,  in  the  other,  thus 
making  double  gains.  To  prove  a  balance,  weigh  an  article  first  in  one  scale 
and  then  in  the  other ;  if  there  is  any  difference  in  the  weight,  the  balance  is 
not  true. 


THE  BALANCE. 


tical  applications  of  the  lever  ?  Show  this  in  the  supposed  case  of  Archimedes. 
205.  When  are  two  bodies  of  equal  weight,  supported  by  the  arms  of  a  lever,  said  to 
be  in  equilibrium  ?  What  is  constructed  on  this  principle  ?  Describe  the  Balance. 
When  great  accuracy  is  required,  how  is  the  beam  balanced  ?  How  sensitive  has  a 
balance  been  made  ?  206.  When  does  the  balance  weigh  correctly  ?  How  do  dishou- 


THE  STEELYARD.  97 

The  true  weight  of  a  body  may  be  determined,  with  a  false  balance, 
by  placing  it  in  either  scale,  balancing  it  with  shot  or  sand,  and  then  remov- 
ing the  body  and  replacing  it  with  weights  till  equilibrium  is  established. 
This  is  called  double  weighing.  It  must  give  the  true  weight ;  for  whatever 
error  is  made  in  the  first  weighing  is  corrected  in  the  second. 

207.  The  Steelyard. — When  bodies  of  unequal  weight 
are  supported  by  the  arms  of  a  lever,  they  will  balance  each 
other  whenever  the  weight  of  the  one  multiplied  into  its 
distance  from  the  fulcrum,  is  equal  to  the  weight  of  the 
other  multiplied  into  its  distance  from  the  fulcrum. 

In  Fig.  91,  let  the  distance  WF  be  Fig.  91. 

one  inch  and  PF  three  inches.    The      W          F  p 


weight  of  the  one  body,  30  pounds,  mul- 

tiplied  into  its  distance  from  the  fulcrum, 

1,  gives  30 ;  the  weight  of  the  other,  10      ' — ' 

pounds,  multiplied  into  its  distance  from  the  fulcrum,  3,  gives  30.    These 

products  being  equal,  the  bodies  will  balance  each  other. 

208.  On  this  principle  the  Steelyard  is  constructed. 
The  Steelyard  is  a  kind  of  balance,  which,  though  not  so 
sensitive  as  the  one  described  above,  answers  very  well  for 
heavy  bodies,  and  is  conveniently  carried,  as  it  requires  but 
a  single  weight,  and  may  be  held  in  the  hand  or  suspended 
anywhere. 

Fig.  92  represents  the  steelyard.  ^         Fig.  92. 

It  is  a  lever  of  unequal  arms ;  from        ^ifl^sMlD f , 

the  shorter  of  which  the  article  to 
be  weighed  is  suspended,  either  di- 
rectly or  in  a  scale-pan,  while  a  con- 
stant weight  is  moved  on  the  longer 
arm  from  notch  to  notch  till  equilib- 
rium is  established.  The  number  THE  STEELYARD. 
at  the  notch  on  which  the  weight 

then  rests,  shows  the  required  weight  in  pounds.  Thus,  15  pounds  is  the 
weight  of  the  sugar-loaf  in  the  Figure.  The  proper  distances  for  the  notches 
are  found  in  the  first  place  by  experiments  with  known  weights  in  the  scale- 
pan. 

To  enable  the  steelyard  to  weigh  still  heavier  objects  without  increasing 

«st  tradesmen  sometimes  defraud  those  with  whom  they  deal  ?  How  may  a  balance 
be  proved  ?  How  may  the  true  weight  of  a  body  be  determined  with  a  false  balance  ? 
What  is  this  process  called?  207.  When  will  bodies  of  unequal  weight  supported  by 
the  arms  of  a  lever  be  in  equilibrium  ?  Illustrate  this  with  Fig.  91.  208.  What  is 
constructed  on  this  principle  ?  Describe  the  Steelyard,  and  the  mode  of  weighing 
with  it  How  are  the  proper  distances  for  the  notches  found  in  the  first  place  ?  With 
5 


98  MECHANICS. 

the  length  of  its  beam,  it  is  often  provided  with  an  additional  hook,  hanging 
in  an  opposite  direction  from  the  other  hook  and  nearer  the  point  from  which 
the  article  to  be  weighed  is  suspended.  When  the  instrument  is  supported 
by  this  hook,  a  new  fulcrum  is  formed,  and  the  weight  is  shown  by  a  new 
row  of  notches  adapted  to  it.  The  greater  the  difference  of  length  between 
the  arms  of  a  steelyard,  the  greater  the  number  of  pounds  that  it  can  weigh. 

209.  When  more  than  two  bodies  are  supported  on  the 
arms  of  a  lever,  if  the  weight  of  each  be  multiplied  by  its 
distance  from  the  fulcrum,  the  lever  will  be  in  equilibrium 
(that  is,  the  bodies  will  balance  each  other)  when  the  sums 
of  the  products  on  the  two  sides  of  the  fulcrum  are  equal. 

j,t(T  Q3  Thus,  in  Fig.  93  equilibrium 

is  maintained,  because  the  prod- 

3ww  W^MW^ •twwgftwwL^?. •. ^ /W777&        ucts  of  the  weights  on  one  side 

[       into  their  distances,  added  to- 

0      ©       ©  ©  Q     gether,   equal  the  sum  of  the 

products  on  the  other : — 

weights     distances  weights     distances 

2X1=2  2X1       =      2 

3X2=6  6X3=     18 
4       X         3     =     12 

Sum  of  products,  20  Sum  of  products,  20 

210;  Practical  Applications. — Familiar  examples  of 
levers  of  the  first  kind  are  found  in  the  scissors  and  pincers ; 
the  rivet  connecting  the  two  parts  being  the  fulcrum,  the 
fingers  the  power,  and  the  thing  to  be  cut  or  grasped  the 
weight.  A  poker  introduced  between  the  bars  of  a  grate 
and  allowed  to  rest  on  one  of  them,  that  purchase  may  be 
obtained  for  stirring  the  fire,  is  a  lever  of  the  first  kind.  So 
is  the  handle  of  a  common  pump. 

Pig.  94  When  children  teeter  on  a  board 

&       balanced  on  a  wooden  horse,  they  use 
a.,4^-1 — gg^jpg      a  lever  of  the  first  kind.    According 
to  the  principles  of  the  lever,  if  one  is 
heavier  than  the  -other,  to  preserve  the 
balance,  he  must  sit  nearer  the  fulcrum,  as  shown  in  Fig.  94 

what  are  some  steelyards  provided,  and  for  what  purpose  ?  What  steelyards  weigh 
the  greatest  number  of  pounds  ?  209.  If  more  than  two  bodies  are  supported  on  the 
arms  of  a  lever,  when  will  they  balance  each  other  ?  Apply  this  principle  in  Fig.  93. 
210.  Give  some  familiar  examples  of  levers  of  the  first  kind.  When  children  teeter 
en  a  board,  what  kind  of  lever  do  they  use  ?  If  one  is  heavier  than  the  other,  where 


BENT   AND    COMPOUND   LEVEES. 


99 


211.  Sent  Levers. — Sometimes  the  arms  of  a  lever  are 
bent,  instead  of  straight.     In  that  case  the  same  principles 
hold  good,  only  that  the  arms  of  the  lever  are  estimated, 
not  by  their  actual  length,  but  by  the  perpendicular  dis- 
tance from  the  fulcrum  to  the  line  of  direction  m  which  the 
power  and  weight  respectively  act. 

As  an  illustration  of  bent  levers  of  the  first  kind, 
we  may  take  the  truck  used  for  moving  heavy  arti- 
cles, represented  in  Fig.  95.  The  axis  on  which  tlie 
wheels  turn  represents  the  fulcrum ;  the  weight  is  ap- 
plied at  W,  and  the  power  at  P.  The  clawed  side  of 
a  hammer,  used  in  drawing  out  nails,  is  also  a  bent 
lever.  The  fixed  point  on  which  the  head  of  the  ham- 
mer rests  is  the  fulcrum ;  the  friction  of  the  nail  is 
the  weight ;  and  the  power  is  applied  at  the  extrem- 
ity of  the  handle. 

212.  Compound  Levers. — Simple  le- 
vers of  the  first  kind  may  be  combined 
into  Compound  Levers. 

213.  In  compound  levers,  equilibrium  is  established  when 
the  power,  multiplied  by  the  first  arms  of  all  the  levers,  is 
equal  to  the  weight  multiplied  by  the  last  arms  of  all  the 
levers. 

Fig.  96. 

F 


A  COMPOUND  LETEE. 

Thus,  in  Fig.  96,  which  represents  a  compound  lever  formed  of  three  sim- 
ple ones,  let  the  long  arm  of  each  lever  be  three  times  the  length  of  its  short 
arm ;  then  1  pound  at  P  will  balance  27  pounds  at  W,  because 
1  pound  X3X3X3  =  27  pounds  X  1  X 1  X  1. 

214.  LEVERS  OP  THE  SECOND  KIND. — In  levers  of  the 
second  kind,  the  relative  position  is 

must  he  sit  to  preserve  the  balance  ?  211.  What  is  meant  by  a  bent  lever  ?  How  are 
the  arms  of  a  bent  lever  estimated  ?  Give  some  familiar  examples  of  bent  levers. 
212.  How  may  simple  levers  of  the  first  kind  be  combined?  213.  When  is  equilib- 
rium established  in  a  compound  lever  ?  Illustrate  this  with  Fig.  96.  214.  In  levers 


100 


MECHANICS. 


POWER  WEIGHT  FDLCRUM      OR     FULCRUM   WEIGHT  POWER. 


Fig.  97. 


Fig.  97  shows  how  the  crow-bar  may 
be  used  as  a  lever  of  the  second  kind. 
The  power  is  applied  at  the  handle ;  the 
fulcrum  is  at  the  other  end,  and  the 
weight  to  be  moved  is  between  them. 

215.  The  nearer  the  weight 
is  to  the  fulcrum  the  greater 
the  advantage  gained,  and  con- 
sequently the  greater  the  space 
that  P  will  have  to  pass  through 

in  moving  W  a  given  distance.     This  principle  is  stated  in 

the  following 

Law. —  With  levers  of  the  second  "kind,  intensity  of  force 

is  gained,  and  time  is  lost,  in  proportion  as  the  distance 

between  the  power  and  the  fulcrum  exceeds  the  distance 

between  the  weight  and  the  fulcrum. 

Thus,  in  Fig.  97,  if  the  distance  P  F  be  five  times  as  great  as  W  F,  a  pres- 
sure of  10  pounds  at  P  will  counterbalance  a  weight  of  50  pounds  at  W,  and 
move  any  thing  under  50  pounds ;  while,  for  every  inch  that  W  is  moved,  P 
will  have  to  move  five  inches  in  the  same  direction. 

216.  Practical  Applications. — The 
common  chipping-knife,  used  by  apothe- 
caries, and  represented  in  Fig.  98,  is  a 
familiar  illustration  of  levers  of  the  sec- 
THE  CHIPPING-KNIFE.  on(i  ^nfl.  The  knife  is  fastened  at  one 
end,  F,  which  thus  becomes  the  fulcrum ;  the  hand  is  ap- 
plied, as  the  power,  at  the  other  end,  P ;  and  the  substance 
to  be  cut  is  the  resistance,  or  weight,  between  them.  Nut- 
crackers and  lemon-squeezers  work  on  the  same  principle, 
and  are  levers  of  the  second  kind. 

A  door  turned  on  its  hinges,  and  an  oar  used  in  rowing, 
are  also  examples  of  this  kind  of  lever.  In  the  former  case, 
the  hinge  is  the  fulcrum ;  the  hand  applied  at  the  knob  is 
the  power ;  and  the  weight  of  the  door,  which  may  be  re- 

of  the  second  kind,  what  is  the  relative  position  of  the  three  Important  points  ?  How 
may  the  crow-bar  be  used  as  a  lever  of  the  second  kind  ?  215.  What  is  the  law  of  levers 
of  the  second  kind  ?  Apply  this  in  Fig.  97.  216.  What  familiar  articles  will  serve  as 
Illustrations  of  levers  of  the  second  kind  ?  Show  how  a  door  turned  on  its  hinges  is 


Fig.  98. 


LEVERS   OF  THE  THIRD   KIND.  101 

garde  d  as  concentrated  in  its  centre  of  gravity  somewhere 
between  the  two,  is  the  resistance.  In  the  latter  case,  the 
point  at  which  the  oar  enters  the  water  is  the  fulcrum ;  the 
rower's  hand  is  the  power ;  and  the  weight  of  the  boat, 
acting  at  the  row-lock,  is  the  resistance.  According  to  the 
law  laid  down  in  §  215,  the  further  from  the  row-lock  we 
grasp  the  oar,  the  more  easily  we  overcome  the  resistance 
and  produce  motion. 

217.  Two  persons  carrying  a  weight  suspended  from  a  stick  between  them, 
use  a  double  lever  of  the  second  kind.  Power  is  applied  at  each  end,  and 
each  end  in  turn  becomes  the  fulcrum  to  the  other,  the  weight  resting  on 
some  intermediate  point.  The  relation  of  the  power  at  one  end  to  the  weight 
is  governed  by  the  same  law  as  that  of  the  power  at  the  other  end ;  and  there- 
fore the  weight,  to  be  divided  equally,  must  be  suspended  from  the  middle  of 
the  stick.  If  it  is  not  so  suspended,  the  man  who  is  nearer  the  weight  car- 
ries more  than  the  other  in  proportion  as  he  is  nearer. 

Thus,  let  a  12-pound  weight,  W,  be  suspended  Fig.  99. 

from  a  bar  three  feet  long,  at  a  distance  of  one  foot     -A , B 

from  A  and  two  feet  from  B.    Then  A  will  carry  A -w 

two-thirds  of  the  weight,  and  B  one-third.     On 

this  principle,  when  it  is  desired  that  one  of  the  horses  harnessed  to  a  car- 
riage should  draw  more  than  the  other,  it  is  necessary  only  to  make  the  arm 
of  the  whiffle-tree  to  which  he  is  attached  proportionally  shorter. 

Fig.  100  shows  how  a  weight  may  be  equally  p     Fig.  100. 

distributed  between  three  persons.     B,  being       &/— 
twice  as  far  from  E  as  D  is,  bears  one-third  of  the    A/ 
weight,  W ;  while  A  and  C,  at  the  extremities 
of  the  equal-armed  lever  ADC,  bear  equal  portions  of  the  remaining  two- 
thirds,  or  one-third  each. 

218.  LEVERS  OP  THE  THIRD  KIND. — In  le-        Fig.  101. 
vers  of  the  third  kind,  the  relative  position  is 

FULCRUM  POWER  WEIGHT  OR  WEIGHT  POWER  FULCRUM. 

The  forceps,  represented  in  Fig.  101,  is  a  lever  of  the 
third  kind.  The  two  sides  unite  at  one  end  to  form  the  ful- 
crum ;  the  article  to  be  grasped  is  the  weight ;  and  the  fin- 
gers, applied  between  the  two,  constitute  the  power. 

219.  Levers  of  the  third  kind,  unlike  those 
before  described,  involve  a  mechanical  disad- 

a  lever  of  the  second  kind.  Show  how  an  oar  acts  as  a  lever  of  the  second  kind. 
217.  When  two  persons  carry  a  weight  suspended  from  a  stick  between  them,  what 
kind  of  a  lever  do  they  use  ?  Where  is  the  fulcrum  ?  To  be  equally  divided,  where 
must  the  weight  be  suspended  ?  If  the  weight  does  not  hang  from  the  middle  of  the 


102  MECHANICS. 

vantage ;  that  is,  to  produce  equilibrium,  the  power  must 
always  be  greater  than  the  weight. 

Law.  —  With  levers  of  the  third  kind,  intensity  of  force 
is  lost,  and  time  is  gained,  in  proportion  as  the  distance 
from  the  weight  to  the  fulcrum  exceeds  the  distance  from, 
the  power  to  the  fulcrum. 

Thus,  in  Fig.  101,  if  F  W  be  three  times  as  great  as  FP,  it  will  require  a 
power  of  three  pounds  at  P  to  counterbalance  a  resistance  of  one  pound  at 
W.  Levers  of  this  class,  therefore,  are  never  used  when  great  power  is  re- 
quired, but  only  when  a  slight  resistance  is  to  be  overcome  with  great  ra- 
pidity. 

220.  Practical  Applications. — The  sugar-tongs,  which 
resembles  in  shape  the  forceps  above  described,  is  a  familiar 
example  of  the  third  kind  of  lever.  So  is  the  fire-tongs ; 
and  hence  the  difficulty  of  raising  heavy  pieces  of  coal  with 
this  instrument,  particularly  when  the  hand  is  applied  near 
the  rivet  or  fulcrum. 

The  sheep-shears  is  another  lever  of  the  third  kind,  admirably  adapted  to 
the  work  it  performs ;  because  the  wool,  being  flexible,  has  to  be  cut  rapidly, 
while  it  does  not  require  any  great  degree  of  force. 

A  door  becomes  a  lever  of  the  third  kind  when  one  attempts  to  move  it  by 
pushing  at  the  edge  near  the  hinges.  The  mechanical  disadvantage  is  shown 
by  the  great  strength  required  to  move  it  when  the  power  is  there  applied. 
So,  when  a  painter  attempts  to  raise  a  ladder  lying  on  the  ground  with  its 
bottom  against  a  wall,  by  lifting  the  top  and  walking  under  it  grasping  round 
after  round  in  succession,  he  experiences  great  difficulty  as  he  approaches 
the  bottom,  because  the  ladder,  when  he  passes  its  centre  of  gravity,  becomes 
a  lever  of  the  third  kind. 

Fig.  102.  ^^        Nature  uses  levers  of  the 

third  kind  in  the  bones  of 
animals.     The  fore-arm  of  a 
man,   represented    in    Fig. 
HTTMAN  ARM  AND  HAND.  1 02,will  serve  as  an  example. 


stick,  which  man  will  carry  the  more  ?  Illustrate  this  with  Fig.  99.  How  may  one 
of  the  horses  harnessed  to  a  carriage  be  made  to  draw  more  than  the  other  ?  How 
may  a  weight  be  equally  distributed  between  three  persons  ?  218.  In  levers  of  the 
third  kind,  what  is  the  position  of  the  three  important  points?  What  instrument  is 
an  example  of  the  third  kind  of  levers  ?  219.  To  produce  equilibrium  in  the  third  kind 
of  levers,  what  is  necessary?  State  the  law  for  levers  of  the  third  kind.  Illustrate 
this  with  Fig.  101.  220.  What  common  articles  are  levers  of  the  third  kind  ?  What 
is  said  of  the  sheep-shears  ?  When  does  a  door  become  a  lever  of  the  third  kind  f 


THE   WHEEL   AND   AXLE. 


103 


The  fulcrum,  F,  is  at  the  elbow-joint ;  the  biceps  muscle,  descending  from 
the  upper  part  of  the  arm  and  inserted  near  the  elbow  at  P,  operates  as  the 
power ;  while  the  weight,  "W,  rests  on  the  hand.  If  the  distance  F  W  be  15 
times  as  great  as  F  P,  it  will  take  a  power  of  15  pounds  at  P  to  counterbal- 
ance one  pound  at  W ;  and  when  the  arm  is  extended,  the  disadvantage  is 
still  greater,  in  consequence  of  the  muscle's  not  acting  perpendicularly  to  the 
bone,  but  obliquely. 

This  accounts  for  the  difficulty  of  holding  out  a  heavy  weight  at  arm's 
length.  In  proportion  as  power  is  lost,  however,  quickness  of  motion  is 
gained ;  a  very  slight  contraction  of  the  muscle  moves  the  hand  through  a 
comparatively  large  space  with  great  rapidity.  Here,  as  in  all  the  works  of 
creation,  the  wisdom  of  Providence  is  shown  in  exactly  adapting  the  part  to 
the  purpose  for  which  it  is  designed.  "With  so  many  external  agents  at  his 
command,  man  does  not  need  any  great  strength  of  his  own ;  quickness  of 
motion  is  much  more  necessary  to  him,  and  this  the  structure  of  his  arm 
ensures. 

The  'Wheel  and  Axle. 

221.  The  Wheel  and  Axle  is  the  second  of  the  simple 
mechanical  powers.    It  consists  of  a  Wheel  attached  to  a 
cylinder,  or  Axle,  in  such  a  way  that  when  set  in  motion 
they  revolve  around  the  same  axis. 

222.  In  the  simplest  form  of  the  wheel  and  axle,  the 
power  is  applied  to  a  rope  passing  round  the  wheel,  while 


the  weight  is   attached  to    another 
rope  passing  round  the  axle. 

This  form  of  the  machine  is  shown  in  Fig.  103. 
C  D  is  a  frame ;  B  is  the  wheel ;  A  is  the  axle, 
attached  to  the  frame  at  its  extremities  E  and  F 
by  gudgeons,  or  iron  pins,  on  which  it  turns.  P 
is  the  power,  and  W  is  the  weight: 

223.  The  wheel  and  axle  is  simply 
a  revolving  lever  of  the  first  kind. 
One  application  of  the  lever  can  not 
move  a  body  any  great  distance ;  but, 
by  means  of  the  wheel  and  axle,  the 
action  of  the  lever  is  continued  unin- 


Fig.  103. 


THE  WHEEL  AND  AXLB. 


Under  what  circumstances  does  a  ladder  become  a  lever  of  the  third  kind  ?  In  what 
does  Nature  use  levers  of  the  third  kind?  Show,  by  Fig.  102,  how  the  fore-arm  is  a 
lever,  and  point  out  the  relation  between  power  and  weight.  How  is  the  wisdom  of 
Providence  shown,  in  making  the  arm  such  a  lever  ?  221.  What  is  the  second  sim- 
ple mechanical  power  ?  Of  what  does  the  Wheel  and  Axle  consist  ?  222.  In  the  sim- 
plest form  of  this  machine,  how  ia  the  power  applied,  and  how  the  weight  ?  Illus- 


104  MECHANICS. 

terruptedly.  This  machine  has  therefore  been  called  the 
perpetual  or  endless  lever. 

224.  The  wheel  and  axle  must  turn  round  their  common  axis  in  the  same 
time.  In  each  revolution,  a  length  of  rope  equal  to  the  wheel's  circumfer- 
ence is  pulled  down  from  the  wheel,  while  only  as  much  rope  is  wound  round 
the  axle  as  is  equal  to  the  axle's  circumference.  There  is,  therefore,  a  loss 
of  time,  greater  or  less  according  as  the  circumference  of  the  wheel  exceeds 
that  of  the  axle ;  but,  by  the  law  of  Mechanics  already  stated,  there  must  be 
a  corresponding  gain  of  power. 

/  Viewing  the  wheel  and  axle  as  a  lever  of  the  first  kind,  we  have  the  cir- 
cumference of  the  wheel  for  the  long  arm,  and  that  of  the  axle  for  the  short 
arm.  If  the  diameters  of  the  wheel  and  the  axle  are  given  instead  of  their 
circumferences,  they  may  be  taken  for  the  two  arms ;  and  so  with  the  radii, 
if  they  are  given.  In  practice,  an  allowance  of  10  per  cent,  of  the  weight 
must  be  made  for  the  stiffness  of  the  ropes  and  the  friction  of  the  gudgeons. 
— From  these  principles  is  deduced  the  following  law  : — 

225.  LAW  OP  THE  WHEEL  AND  AXLE. —  With  the  wheel 
and  axle,  intensity  offeree  is  gained,  and  time  is  lost,  in 
proportion  as  the  circumference  of  the  wheel  exceeds  that  of 
the  axle. 

Thus,  in  Fig.  103,  if  the  circumference  of  the  wheel  B  is  five  feet  and  that 
of  the  axle  A  one  foot,  a  power  of  40  pounds  at  P  will  counterbalance  a  weight 
of  200  pounds  at  W,  and  of  course  lift  any  thing  under  200  pounds. 

226.  DIFFERENT  FORMS. — The  wheel  and  axle  is  exten- 
sively used,  and  assumes  a  variety  of  forms. 

Fig.  104.  Instead  of  having  a  rope  attached  to  it,  the 

wheel  is  often  provided  with  projecting  pins,  as 
shown  in  Fig.  104,  to  which  the  hand  is  directly 
applied.  This  form  of  the  machine  is  used  in 
the  pilot-houses  of  steamboats  for  moving  the 
rudder.  In  calculating  the  advantage  in  this 
case,  instead  of  the  circumference  of  the  wheel 
we  must  take  the  circumference  of  the  circle 
described  by  the  point  to  which  the  hand  is  ap- 
plied. 

A  still  more  common  form,  much  used  in  drawing  water  from  wells  and 
loaded  buckets  from  mines,  is  shown  in  Fig.  105.  Instead  of  a  wheel,  we 

trate  this  with  Fig.  103.  223.  What  has  the  wheel  and  axie  been  called,  and  why? 
224.  Explain  the  operation  of  the  wheel  and  axle,  and  show  how  great  the  loss  of  time 
and  gain  of  power  will  be.  Viewing  the  wheel  and  axle  as  a  lever,  what  is  the  long 
arm  ?  What  is  the  short  arm  ?  What,  besides  the  circumference,  may  be  taken  as 
the  arms  of  the  lever  ?  What  allowance  must  be  made  in  practice  ?  225.  State  the  law 
of  the  wheel  and  axle.  Illustrate  this  law  with  Fig.  103.  226.  Describe  the  form  of 


CAPSTAN  AND  WINDLASS. 


105 


Fig.  105. 


Fig.  107. 


have  here  a  Winch,  or  handle,  attached  to  the  axle. 
In  this  case,  to  calculate  the  advantage  gained,  we 
must  compare  the  circle  described  by  the  extrem- 
ity of  the  handle  (shown  in  the  Figure  by  a  dotted 
line)  with  the  circumference  of  the  axle. 

Fig.  106.  FiS-  106  shows  a 

-.....,  third  form  of    the 

wheel     and     axle. 
Here  the  axle  A  is 

vertical,  instead  of  horizontal.  A  bar  insert- 
ed in  its  head,  at  the  extremity  of  which  the 
hand  is  applied,  takes  the  place  of  the  wheel. 
If  the  circumference  of  A  is  3  feet  and  the 
circle  described  by  P  is  12  feet,  a  power  of  1 
pound  at  P  will  counterbalance  a  weight  of  4 
pounds  at  W. 

227.  The  Capstan. — The  Capstan  (see  Fig.  107)  is  a  fa- 
miliar example  of  this  form  of  the  wheel  and  axle.    It  is 
used  by  sailors  for  warping  vessels  up  to  a 

dock,  raising  anchors,  &c. ;  and  consists  of  a 
massive  piece  of  timber,  round  which  a  rope 
passes.  This  is  surmounted  by  a  circular  head, 
perforated  with  holes,  into  which,  when  the  in- 
strument is  to  be  used,  strong  bars,  called 
handspikes,  are  inserted.  Several  men  may  work  at  each 
handspike,  pushing  it  before  them  as  they  walk  round  the 
capstan.  The  handspikes  act  on  the  principle  of  the  lever. 
The  longer  they  are,  therefore,  the  more  easily  the  men 
overcome  the  resistance,  but  the  further  they  have  to  walk 
in  doing  it. 

228.  The  Windlass. — This  is  a  similar  form  of  the  wheel 
and  axle,  used  on  shipboard  for  various  purposes. 

The  windlass  is  not  vertical,  like  the  capstan,  but  horizontal  or  parallel 
to  the  deck.  It  is  a  round  piece  of  timber,  supported  at  each  end,  and  per- 
forated with  rows  of  holes.  Pushing  against  handspikes  inserted  in  these 

the  wheel  and  axle  used  in  the  pilot-houses  of  steamboats.  In  calculating  the  advan- 
tage in  this  case,  what  must  we  substitute  for  the  circumference  of  the  wheel  ?  De- 
scribe the  form  of  the  machine  used  in  drawing  water  from  wells.  How  is  the  ad- 
vantage ascertained  in  this  case?  Describe  a  third  form  of  the  wheel  and  axle, 
exhibited  in  Fig.  106.  227.  What  machine  is  a  familiar  example  of  this  third  form? 
Tor  what  is  the  Capstan  used  ?  Of  what  does  it  consist  ?  How  is  it  worked  ?  How  do 
the  handspikes  act  ?  228.  What  similar  instrument  is  often  substituted  for  the  cap- 


TUB  CAPSTAN. 


106 


MECHANICS. 


holes,  the  boatmen  turn  the  barrel  of  the  windlass  halfway  over.  It  ia  held 
there  by  a  suitable  apparatus,  till  the  handspikes  are  removed  and  put  in  a 
new  row  of  holes,  when  the  process  is  repeated.  The  windlass  acts  on  the 
same  principle  as  the  capstan,  but  is  less  convenient,  on  account  of  the  man- 
ner in  which  the  force  is  applied,  and  the  necessity  of  removing  the  hand- 
spikes to  new  holes  from  time  to  time. 

229.  Wheels  enter  largely  into  machinery.     The  modes 
of  connecting  them  will  be  considered  hereafter. 


Fig.  108. 


The  Pulley. 

230.  The  Pulley  is  the  third  of  the  simple  mechanical 

powers.  It  consists  of  a  wheel  with  a 
grooved  circumference,  over  which  a  rope 
passes,  and  an  axis  or  pin,  round  which 
the  wheel  may  be  made  to  turn.  The 
ends  of  the  axis  are  fixed  in  a  frame 
called  a  Hock. 

Fig.  108  gives  a  view  of  the  pulley.    A  represents 
the  block,  B  the  axi«,  and  C  the  wheel.    Round  the 
groove  in  the  wheel  passes  a  rope,  at  one  end  of 
which  the  power  acts,  while  the  weight  is  attached 
THE  PULLEY.  to  the  other. 

231.  KINDS  OP  PULLET. — Pulleys  are  of  two  kinds, — 
Fixed  and  Movable. 

232.  Fixed  Pulleys.— A  Fixed  Pulley  is 
one  that  has  a  fixed  block. 

Fig.  109  represents  a  fixed  pulley.  The  block  is  at- 
tached to  a  projecting  beam.  P  is  the  power,  and  W  the 
weight.  For  every  inch  that  P  descends,  W  ascends  the 
same  distance.  There  is,  therefore,  no  loss  of  time,  and  no 
gain  in  intensity  of  force.  One  pound  at  P  will  just  coun- 
terbalance one  pound  at  W. 

233.  In  this  rule,  as  well  as  all  the  others  pertaining 
to  the  Mechanical  Powers,  it  must  be  remembered  that 
friction  is  not  taken  into  account.  In  the  case  of  the  pul- 
ley, in  consequence  of  the  stiffness  of  the  rope  and  the 
friction  of  the  pin,  an  allowance  of  20  per  cent,  of  the 
weight,  and  often  more,  must  be  made  in  practice. 


Fig.  109. 


A  I! 


FIXED  PULLET. 


stan  ?  How  does  the  "Windlass  differ  from  the  capstan  ?  Of  what  does  the  windlass 
0onsist  ?  How  is  it  worked  ?  What  makes  it  less  convenient  than  the  capstan  ? 
229.  What  is  said  of  wheels  ?  280.  What  is  the  third  simple  mechanical  power  ?  Of 


FIXED   PULLEYS. 


107 


Fig.  110. 


234.  Though  no  power  is  gained  with  the  fixed  pulley, 
it  is  frequently  used  to  change  the   direction  of  motion. 
The  sailor,  instead  of  climbing  the  mast  to  hoist  his  sails, 
stands  on  deck,  and  by  pulling  on  a  rope  attached  to  a 
pulley  raises  them  with  far  less  difficulty.     With  equal  ad- 
vantage the  builder  uses  a  fixed  pulley  in  raising  huge 
blocks  of  stone  or  marble,  and  the  porter  in  hoist- 
ing heavy  boxes  to  the  lofts  of  a  warehouse. 

235.  "With  two  fixed  pulleys,  horizontal  motion 
may  be  changed  into  vertical ;  horses  are  thus  en- 
abled to  hoist  weights,  as  shown  in  Fig.  84. 

236.  Fig.  110  shows  how  a  person  may  raise 
himself  from  the  ground,  or  let  himself  down  from 
a  height,  by  means  of  a  fixed  pulley.     In  lofty 
buildings  an  apparatus  of  this  kind  is  sometimes 
rigged  near  a  window,  to  furnish  means  of  escape 
in  case  of  fire. 

237.  Movable  Pulleys. — A  Movable  Pulley 
is  one  that  has  a  movable  block. 

Fig.  Ill  represents  a  movable  pulley.    A  is  the  wheel. 

One  end  of  the  rope  is  fastened  to  a  support  at  D,  while 

the  power  is  applied  to  the  other  at  P. 

238.  To  raise  the  weight  a  given  distance  with  the 

movable  pulley,  the  hand  must  be  raised  twice  that  dis- 
tance. Time,  therefore,  being  lost  in  the 
proportion  of  2  to  1,  the  intensity  of  the 
force  is  doubled.  A  power  of  one  pound 
at  P  will  counterbalance  two  pounds  at 
"W,  and  raise  anything  under  two  pounds. 

239.    A  movable   pulley  is 
seldom  used  alone.     It  is  gen-   ™VABLE  PI™. 
W    erally  combined  with  a  fixed  pulley,  as  shown 
in  Fig.  112.     No   additional  power  is  thus 


Fig.  111. 


Fig.  112. 


•what  does  the  Pulley  consist  ?  What  is  the  Block  ?  Point  out  the  parts  of  the  pul- 
ley in  Fig.  103.  231.  How  many  kinds  of  pulleys  are  there  ?  232.  What  is  a  Fixed 
Pulley  ?  Point  out  the  parts  in  the  Figure.  What  is  the  gain  with  this  pulley? 
233.  What  allowance  must  be  made  for  friction  in  the  case  of  the  pulley  ?  234.  If  flo 
power  is  gained  by  the  use  of  the  fixed  pulley,  why  is  it  used  ?  Give  example^ 
235.  How  may  horizontal  motion  be  changed  into  vertical  ?  236.  What  does  Fig.  110 


108 


MECHANICS. 


rig.  113.  gained  ;  on  the  contrary,  there  is  a  loss,  thfr 
friction  of  two  pulleys  being  double  that  of  one. 
But  this  loss  is  more  than  counterbalanced  by 
the  greater  convenience  of  pulling  downward. 

240.  When  a  high  degree  of  force  is  required,  several  mov* 
able  and  fixed  pulleys  may  be  combined,  as  represented  in 
Fig.  113.  A  and  B  are  fixed  pulleys ;  C  and  D  are  movable 
ones,  from  the  block  of  which  the  weight  W  is  suspended. 
One  end  of  the  rope  is  attached  to  the  lower  extremity  of  the 
fixed  block,  F ;  to  the  other  end  the  power  is  applied,  after  the 
rope  has  passed  in  succession  over  each  of  the  four  pulleys. 

To  move  W  an  inch  with  this  combination,  each  length  of 
rope  must  be  shortened  an  inch,  and  therefore  P  must  move 
as  many  inches  as  there  are  lengths  of  rope.  Since  there  are 
two  lengths  of  rope  for  each  movable  pulley,  we  may  lay  down 
the  following  law  : — 

241.  Law  of  Movable  Pulleys. —  With  mov- 
able pulleys^  a  power  will  balance  a  weight  as  many  times 
greater  than  itself  as  twice  the  number  of  movable  pulleys 
employed* 

In  Fig.  113,  a  power  of  1  pound  will  balance  a  weight  of  4  pounds.  If 
three  movable  pulleys  were  used,  1  pound  at  P  would  balance  6  pounds  at  W ; 
if  four  were  used,  8  pounds,  &c.  Friction,  however,  nullifies  much  of  this 
gain. 

242.  White's  Pulley. — To  lessen  the  friction,  when  a 
number  of  pulleys  are  required,  the  wheels  are  made  to 
turn  on  the  same  axis.  This  is  effected  by  having  but  one 
block  for  all  the  upper  pulleys,  and  one  for  the  lower ; 
grooves  being  cut  in  each,  to  take  the  place  of  separate 
wheels.  The  friction  in  each  block  is  thus  reduced  to  that 
of  a  single  wheel.  This  system  is  called,  from  its  inventor, 
White's  Pulley. 

Fig.  114  gives  a  front  and  a  side  view  of  White's  Pulley.    A  is  the  fixed 

stow  ?  For  what  is  an  apparatus  of  this  kind  sometimes  used  ?  237.  What  is  a  Mov- 
able Pulley?  Describe  it  with  Fig.  111.  23S.  To  move  a.  weight  a  given  distance 
•with  a  movable  pulley,  how  far  must  the  power  travel  ?  What,  then,  is  the  law  of 
this  machine  ?  239.  With  what  is  a  movable  pulley  generally  combined  ?  What  is 
gained  by  this  combination?  240.  Describe  the  combination  of  movable  and  fixed  pul- 
leys represented  in  Fig.  113.  241.  What  is  the  law  of  movable  pulleys?  Apply  this 
law  in  the  case  of  the  pulley  represented  in  Fig.  113.  By  what  is  much  of  this  gain 
minified  ?  242.  When  a  number  of  pulleys  are  required,  how  is  the  friction  lessened? 


MOVABLE   PULLEYS. 


109 


Fig.  114 


block,  with  grooves  of  different  sizes  representing  the  separate  wheels.    B  is 

the  movable  block,  similarly  prepared.    A  single  rope  is  used,  which  is 

fastened  at  one  end  to  the  smallest  fixed 

pulley,  and  acted  on  by  the  power  at  the 

other.     Here  again,  if  friction  is  left  out  of 

account,  the  power  will  counterbalance  a 

weight  as  many  times  greater  than  itself  as 

twice  the  number  of  movable  pulleys.    In 

Fig.  114  there  are  six  movable  pulleys; 

consequently,  with  a  pressure  of  1  pound 

at  P,  equilibrium  will  be  established  when 

"W  is  twice  six,  or  12,  pounds. 

243.  Fig.  115  shows  another 
system  of  movable  pulleys,  each 
of  which  has  a  separate  rope  of 
its  own  attached  at  one  end  to  a 
fixed  support. 

Fig.  115.  To  raise  the  low- 

est pulley,  A,  and  the 
weight  suspended 
from  it  one  inch,  two 
inches  of  its  rope 
must  be  pulled  up. 
This  is  done  by  pull- 
ing up  twice  2,  or  4,  inches  of  B's  rope ;  and  this,  in 
turn,  by  pulling  up  twice  4,  or  8,  inches  of  C's  rope- 
P,  therefore,  must  descend  8  inches,  to  raise  W  one  inch. 
If  there  were  four  movable  pulleys,  P  would  have  to  de- 
scend 16  inches  to  raise  W  one  inch ;  if  5,  32  inches,  and 
so  on, — P's  distance  doubling  for  each  new  pulley  add- 
ed. Hence,  with  this  combination,  the  power  balances  a 
weight  as  many  times  greater  than  itself  as  2  raised  to  the 
power  denoted  ly  the  number  of  movable  pulleys. 

244.  The  pulley  is  so  cheap  and  conve* 
nient  that  it  is  much  used  in  its  simple  forms.  In  com- 
plicated systems,  more  than  half  the  advantage  is  lost  by 
friction  and  the  stiffness  of  the  ropes ;  and  consequently 
such  systems  are  used  only  when  immense  weights  are  to 
be  raised. 


WHITE'S  PULLEY. 


"What  pulley  is  coustructed  on  this  principle?  Describe  White's  Pulley.  With 
White's  Pulley,  what  is  the  gain  ?  243.  Describe  the  system  of  pulleys  represented  in 
Fig.  115.  Explain  its  operation.  What  is  the  gain  with  this  system  ?  244.  What  is 


Fig.  116. 


MECHANICS. 


Inclined  Plane. 

ied  Plane  is  the  fourth  of  the  simple  me- 
lt is  a  plane  surface,  inclined  to  the  ho- 
rizon at  any  angle.     Every  road  not  perfectly  level  is  an 

inclined  plane. 

A  D,  in  Fig.  116,  is  an  inclined  plane, 
of  which  AC  is  the  length,  AB  the 
height,  and  B  C  the  base.  In  theory, 
an  inclined  plane  is  perfectly  smooth 
and  hard.  No  such  surface,  however, 
exists;  and,  therefore,  in  estimating 
THB  INCLINED  PLANK.  the  advantage  of  this  machine  for  prac- 

tical purposes,  allowance  must  be  made  for  friction,  according  to  the  irregu- 
larity or  softness  of  the  surface. 

246.  When  a  body  is  moved  over  a  horizontal  surface,  its  weight  is  sup- 
ported, and  the  resistance  of  the  air  and  friction  are  all  that  have  to  be  over- 
come. When  a  body  is  lifted  perpendicularly,  there  is  no  friction,  but  we 
must  overcome  the  whole  weight  and  the  resistance  of  the  air.  When  a  body 
is  drawn  up  an  inclined  plane,  the  resistance  of  the  air,  friction,  and  a  por- 
tion of  the  weight  must  be  overcome, — more  or  less  of  the  weight  being  sup- 
ported, according  to  the  inclination  of  the  plane.  It  is,  therefore,  harder  to 
move  a  body  up  an  inclined  plane  than  over  a  level  surface,  as  we  know 
by  dragging  a  wagon  up  hill ;  but  it  is  easier  than  to  lift  it  to  the  same 
height. 

247.  Law.  —  With  an  inclined  plane,  intensity  of  force 
is  gained,  and  time  is  lost,  in  proportion  as  its  length  ex- 
ceeds its  height. 

Thus,  in  Fig.  117,  let  the  length  of 
the  plane  A  B  be  12  feet,  and  its  height 
4  feet ;  then  1  pound  at  P  will  counter- 
balance 3  pounds  at  W. 

With  a  given  height,  the  longer  the 
plane  the  easier  it  is  to  raise  an  object 
upon  it.  Hence,  on  steep  mountains, 
the  road  is  not  carried  from  the  bottom 


Fig.  117. 


•aid  of  the  pulley  in  its  simple  forms?  "What  is  said  of  complicated  systems  of  pul- 
leys ?  245.  What  is  the  fourth  simple  mechanical  power  ?  What  is  the  Inclined  Plane?, 
For  what  must  allowance  be  made,  in  estimating  the  advantage  of  the  inclined  plane, 
and  why?  246.  As  regards  the  resistance  to  be  overcome,  show  the  difference  be- 
tween moving  a  body  over  a  horizontal  surface,  lifting  it,  and  drawing  it  up  an  in- 
elined  plane.  247.  What  is  the  law  of  the  inclined  plane  ?  Illustrate  this  law  witk 
Fig.  117.  How  is  the  road  up  a  steep  mountain  frequently  made,  and  why  ?  How 


THE   INCLINED   PLANE.  Ill 

directly  to  the  top,  but  winds  round  the  sides.  Instinct  teaches  a  horse  this 
principle ;  for,  if  left  to  himself  in  ascending  a  hill,  he  does  -not-go  straight 
up,  but  moves  in  a  zigzag  course  from  one  side  of  the  road  to  the  other, -thus 
taking  more  time,  but  making  the  ascent  easier. 

248.  Practical   Applications. — When    hogsheads    or 
heavy  boxes  are  to  be  raised  into  carts  or  pulled  up  a  pair 
of  stairs,  the  work  is  facilitated  by  laying  long  planks,  or 
skids,  in  such  a  way  as  to  form,  an  inclined  plane.     A  piece 
of  board  is  similarly  placed,  if  a  carriage  or  wheelbarrow 
has  to  be  raised  over  a  high  curb-stone. — The  marine  rail- 
way, on  which  ships  of  immense  weight  are  drawn  out  of 
the  water,  to  be  repaired,  is  one  of  the  most  useful  appli- 
cations of  this  machine. 

249.  The  inclined  plane  was  known  to  the  ancients.    It 
is  supposed  that  the  Egyptians  used  it  in  raising  the  huge 
blocks  of  stone  employed  in  the  construction  of  their  pyra- 
mids. 

250.  Law  of  Bodies  rolling  down  an  Inclined  Plane. — 
When  bodies  are  allowed  to  roll  down  an  inclined  plane, 
they  have  a  uniformly  accelerated  motion,  and  attain  the 
same  velocity  by  the  time  they  reach  the  bottom  that  they 
would  have  if  dropped  perpendicularly  from  the  starting 
point. 

A  ball  dropped  from  a  heigftt  of  Q±l/3  feet,  when  it  strikes  the  ground,- has 
a  velocity  of  6&l/3  feet  in  a  second.  If  it  were  allowed  to  roll  from  the  same 
height,  down  an  inclined  surface  a  mile  long,  perfectly  smooth  and  hard,  it 
would  have  the  same  velocity  on  reaching  the  bottom.  The  shorter  the  plane, 
the  less  time  it  would  take  for  its  descent  and  the  sooner  it  would  acquire  the 
Velocity  in  question. 

251.  When  the  perpendicular  height  is  considerable,  objects  rolling  or 
sliding  down  an  inclined  plane  acquire,  near  the  bottom,  a  prodigious  veloci- 
ty. A  remarkable  instance  of  this  was  exhibited  at  a  slide  near  Lake  Lucerne, 
Switzerland,  down  which  fir-trees  were  allowed  to  descend,  from  the  top  of  a 
mountain.  The  slide  was  about  eight  miles  long ;  and,  though  the  descent  was 
but  300  feet  to  a  mile  and  the  road  was  often  circuitous,  the  trees  went  tearing 
along  with  frightful  speed,  performing  the  whole  distance  in  six  minutes. 

4ocs  a  horse  ascending  a  hill  display  his  instinct?  248.  In  what  familiar  cases  is  the 
Inclined  plane  used  ?  What  is  one  of  the  most  useful  applications  of  this  machine  ? 
249.  By  whom  is  the  inclined  plane  thought  to  have  been  used  in  ancient  times? 
850.  What  is  the  law  of  bodies  rolling  down  an  inclined  plane  ?  Illustrate  thi^ 
251.  When  do  bodies  sliding  down  an  inclined  plane  acquire  a  prodigious  velocity? 


112  MECHANICS. 

The  Wedge. 

252.  The  Wedge  is  the  fifth  of  the  simple  mechanical 
powers.     It  appears  in  two  forms,  according  to  the  use  for 
which  it  is  designed. 

253.  FIRST  KIND  OF  WEDGE. — In  its  first  form,  the 
wedge  is  simply  a  solid  and  movable  inclined  plane.     It  is 
used  for  raising  great  weights  a  short  distance,  and  follows 
the  law  of  the  inclined  plane  ;  that  is,  the  power  counterbal- 
ances a  weight  as  many  times  greater  than  itself  as  the 
height  of  the  wedge  is  contained  in  its  length. 

-p.    118  Fig.  118  shows  how  the  wedge  may  be  used 

for  raising  weights.  W  D  is  a  pillar,  so  fixed 
that  it  can  not  move,  except  perpendicularly 
upward.  A  B  is  a  wedge  resting  on  its  base. 
The  sharp  edge  being  brought  near  the  ex- 
tremity of  the  pillar,  power  is  applied  to  the 
side  B  C.  W  must  rise,  as  it  can  not  move  in 
any  other  direction.  By  driving  the  wedge 
under  to  C,  the  pillar  is  raised  the  distance 
BC. 

Fig.  119.  A  more  common  mode  of  raising  bodies 

with  this  machine  is  shown  in  Fig.  119.  A  and 
B  are  similar  wedges.  Simultaneous  blows 
are  given  them  at  A  and  B  in  opposite  direc- 
tions with  heavy  mallets,  and  the  weight  W  is 
slowly  raised.  The  same  power  must  be  ap- 
plied to  each  as  if  it  acted  alone.  Twice  as 
much  power,  therefore,  is  required  as  when 

but  one  wedge  is  used,  but  the  weight  is  raised  twice  as  high  in  a  given  time. 
254.  Thus  applied,  the  wedge  is  an  efficient  and  useful  machine.  It  raises 
immense  weights,  though  to  no  great  distance.  With  its  aid,  ships  are 
brought  up  on  the  dry  dock,  and  houses  thrown  out  of  line  by  the  sinking  of 
their  foundations  are  restored  to  the  perpendicular.  "Wedges  are  also  used 
in  extracting  oil  from  seeds.  The  seeds  are  placed  between  immovable  tim- 
bers, in  bags  that  allow  the  liquid,  as  it  is  pressed  out,  to  ooze  through.  Be- 
tween the  bags  are  then  inserted  wedges,  which  are  gradually  driven  in.  So 
intense  is  their  pressure  that  every  particle  of  oil  is  extracted,  and  the  seeds, 
when  taken  out,  are  found  mashed  together,  into  a  dense  solid  mass. 

What  instance  of  this  is  mentioned  ?  252.  What  is  the  fifth  mechanical  power  ?  In 
how  many  forms  does  the  wedge  appear  ?  253.  Describe  the  first  kind  of  wedge. 
For  what  is  it  used  ?  What  law  does  it  follow  ?  Describe  the  operation  of  this  sort 
of  wedge  with  Fig.  118.  What  is  the  more  common  mode  of  raising  bodies  with  this 


THE   WEDGE.  113 

255.  Familiar  Applications.— Chisels  and  other  tools  sloped,  or  chamfered, 
as  it  is  called,  on  only  one  side,  are  familiar  examples  of  this  sort  of  wedge. 
The  longer  the  chamfered  part  in  proportion  to  its  thickness,  the  more  easily 
the  chisel  overcomes  the  resistance  of  the  wood  into  which  it  is  driven. 

256.  SECOND  KIND  OF  WEDGE. — The  second  kind  of 
wedge  (see  Fig.  120)  has  the  shape  of  two  inclined  planes 
united  at  their  bases.     It  is  used  for  Fig.  120. 
splitting  timber  and  rending  rocks 

in  quarries. 

The  resistance  overcome  by  the  wedge,  when 
thus  used,  is  the  cohesion  of  the  substance  to  be 
split.  As  long  as  the  wedge  is  merely  pressed  THE  WEDGE. 

against  this  substance,  little  or  nothing  is  effected ;  but,  when  driven  in  with 
blows,  it  becomes  a  highly  useful  instrument.  When  once  forced  in,  it  is 
prevented  from  receding  by  the  friction  of  the  wood  against  its  sides.  Thus 
every  blow  begins  to  act  where  the  preceding  blow  left  off  acting. 

257.  Advantage  gained. — The  exact  advantage  gained 
by  this  sort  of  wedge  when  driven  in  by  blows,  can  not  be 
computed.     The  percussion  gives  such  a  shock  to  the  par- 
ticles that  they  open  a  little  in  advance  of  the  wedge,  as 
shown  in  Fig.  120,  and  readily  allow  it  to  enter. 

The  only  law  we  can  lay  down  is  this :  —  With  a  given 
thickness ,  the  longer  a  wedge  is^  the  more  easily  it  pene- 
trates. 

258.  Familiar  Applications. — Knife  and  razor  blades, 
the  heads  of  axes  and  hatchets,  nails,  and  all  cutting  in- 
struments chamfered  on  both  sides,  are  examples  of  this 
kind  of  wedge.     Pins  and  needles  may  be  looked  upon  as 
wedges  with  an  infinite  number  of  sides.     In  all  these,  the 
longer  the  instrument  in  proportion  to  its  thickness,  the 
greater  the  advantage  gained. 

259.  In  seeking  to  increase  the  advantage  of  the  wedge  by  lengthening  it, 
care  must  be  taken  not  to  make  it  too  long.  A  slender  tool  will  answer  for 

machine  ?  What  is  said  of  the  power  In  this  case  ?  254  For  what  is  the  first  kind  of 
wedge  used?  Describe  the  mode  of  extracting  juices  from  seeds.  255.  What  tools 
are  examples  of  this  kind  of  wedge  ?  On  what  does  the  ease  with  which  they  over- 
come the  resistance  depend  ?  256.  Describe  the  second  kind  of  wedge.  For  what  is 
it  used  ?  What  sort  of  power  must  be  applied  to  the  wedge,  when  thus  used  ?  What 
prevents  the  wedge  from  receding?  25T.  What  is  said  of  the  advantage  gained  by 
the  wedge  ?  What  is  the  only  law  that  can  be  laid  down  for  this  machine  ?  258.  Men- 
tion some  familiar  examples  of  the  second  kind  of  wedge.  259.  In  the  case  of  the 


114  MECHANICS. 

soft  substances,  but  not  for  hard.  A  carpenter's  chisel,  for  instance,  whose 
chamfered  edges  make  an  angle  of  30  degrees,  would  soon  break  if  used  on 
iron.  When  this  metal  is  to  be  cut,  the  edges  should  make  an  angle  of  60 
degrees,  and  for  copper  at  least  80. 

The  Screw. 

260.  The  screw  is  the  sixth  and  last  of  the  simple  me- 
chanical powers.     It  consists  of  a  cylinder  with  a  spiral 
Fi"  121     ridge  and  groove  winding  alternately  round  it  in 
A       parallel  curves.     The  portions  of  the  ridge  passing 
successively  from  one  side  of  the  cylinder  to  the 
other  are  called  the  Threads  of  the  screw. 

Fig.  121  represents  a  screw.  If  we  could  unwind  the  threads 
from  the  cylinder,  commencing  at  the  end  A,  we  should  have  a 
continuous  wedge.  The  back  of  this  wedge  is  applied  to  the  cyl- 
inder; and  on  its  thickness  depends  the  distance  between  the 
threads  of  the  screw. 

261.   KINDS   OF  SCKEW. — Screws  are   of  two 
kinds : — 

1.  The  Exterior  or  Convex  Screw,  represented  in  Fig. 

121,  in  which  the  ridge  and  groove  are  on  the  out- 
side of  the  cylinder. 

2.  The  Interior  or  Concave  Screw,  in  which  the  ridge 

and  groove  are  on  what  may  be  regarded  as  the 
inside  surface  of  a  cylinder. 

These  two  forms  are  used  together,  and  are  generally 
called  the  Screw  and  the  Nut.  Every  screw  must  have  a 
nut  grooved  in  such  a  way  as  to  receive  its  ridge. 

262.  Advantage  gained. — The  power  is  applied  at  the 
head  of  the  screw.  The  resistance  is  to  be  overcome  by 
pressure  produced  at  its  other  end.  Every  time  the  screw 
is  turned  once  round  in  the  nut,  it  advances  as  far  as  the 
distance  between  two  of  its  threads,  and  compresses  to  that 


wedge,  what  must  be  avoided  ?  What  difference  is  there  between  a  carpenter's  chisel 
and  one  suitable  for  iron  and  copper?  260.  What  is  the  sixth  mechanical  power? 
Of  what  does  the  Screw  consist  ?  What  is  meant  by  the  Threads  of  the  screw  ?  If 
we  could  unwind  the  threads  from  the  cylinder,  what  would  they  form?  2(51.  How 
many  kinds  of  screws  are  there  ?  Name  and  describe  each.  How  are  these  two  forms 
of  the  screw  used,  and  what  are  they  generally  called  ?  262.  With  the  screw,  how 


THE   SCREW. 


115 


extent  any  fixed  object  against  which  it  is  directed.  "With 
the  screw,  therefore,  the  power  produces  a  pressure  as  many 
times  greater  than  itself,  as  the  circumference  of  the  head 
is  greater  than  the  distance  between  the  threads. 


Here,  again,  however,  friction  lessens  the  ef- 
fect ;  and,  to  gain  greater  power,  a  lever  is  gen- 
erally combined  with  the  screw.  The  mode  of 
doing  this  is  shown  in  Fig.  122,  in  which  S  is 
the  screw  and  L  the  lever. 

In  calculating  the  advantage  in  this  case,  in- 
stead of  the  circumference  of  the  head  take  the 
circle  described  by  the  point  of  the  lever  at  which 
the  hand  is  applied.  In  Fig.  122,  let  the  distance 
between  the  threads  be  1  inch  and  the  dotted 
circle  100  inches;  then  (friction  being  left  out 
of  account)  a  power  of  1  pound  at  the  extremity 
of  the  lever  will  produce  a  pressure  of  100  pounds 
at  the  lower  end  of  the  screw. 

263.  BOOK-BINDER'S  PRESS. 
— The  Book-binder's  Press,  rep- 
resented in  Fig.  123,  exhibits 
one  of  the  most  useful  and  con- 
venient modes  of  applying  the 
tjcrew. 

S  S  is  a  screw,  playing  in  a  station- 
ary nut  in  the  head  of  the  press.  At- 
tached to  the  screw  near  its  bottom  are 
two  bars  at  right  angles  to  each  other, 
at  the  extremities  of  which  the  hand  is 
applied  when  the  press  is  to  be  worked. 
Still  greater  leverage  is  obtained  by  ap- 
plying the  power  at  the  end  of  a  bar,  P, 
introduced  successively  into  holes  in  the 
extremities  of  the  cross-pieces,  as  in 
working  the  windlass.  A  fall  or  platen, 
B  B,  is  attached  to  the  screw,  in  such  a 


Fig.  122. 


THE  SCREW  AND  NTTT. 


Fig.  123. 


BOOK-BINDER'S  PKESS. 


way  that  it  does  not  turn  as  the  screw  revolves,  but  must  rise  or  descend  with 
it.  Between  this  fall  and  the  bed  of  the  press,  D,  the  books  to  be  pressed  are 

great  a  pressure  does  the  power  produce  ?  In  practice,  what  lessens  the  effect  ?  How- 
is  greater  power  obtained  ?  "When  a  lever  is  combined  with  the  screw,  how  may  we 
find  the  advantage  gained  ?  Illustrate  this  with  Fig.  122.  263.  What  machine  ex- 
hibits a  useful  application  of  the  screw?  Describe  the  book-binder's  press.  How  i» 


116 


MECHANICS. 


placed.    Here  again,  to  obtain  the  advantage,  divide  the  circumference  of  the 
circle  described  by  P,  by  the  distance  between  the  threads. 

264.  Screws,  applied  in  this  or  some  similar  way,  are 
extensively  used  when  a  great  and  continued  pressure  is 
required  within  a  small  space.     Cotton  is  compressed  into 
bales,  juices  are  extracted  from  fruit,  coins  are  stamped, 
and  houses  are  raised  from  their  foundations,  with  the  aid 
of  the  screw. 

265.  HUNTER'S  SCREW. — When  intense  pressure  is  re- 
quired, the  threads  of  the  screw  have  to  be  so  close  to- 
gether that  they  are  necessarily  thin  and  liable  to  break. 
To  prevent  this,  an  ingenious  contrivance,  called  after  its 
inventor  Hunter's  Screw,  is  used. 

Hunter's  Screw  consists  of  two  screws,  working  one 
within  the  other,  in  such  a  way  that  as  the  larger  descends 
the  smaller  ascends,  though  not  quite  so  far.  The  differ- 
ence between  the  respective  distances  of  the  threads  in  the 
two  screws  determines  how  far  on  the  whole  the  screw  ad- 
vances. With  Hunter's  Screw,  therefore,  the  power  pro- 
duces a  pressure  as  many  times  greater  than  itself,  as  the 


Fig.  124. 


UP 
w 

HTTNTEE'S  SCBETT. 


difference  between  the  respective 
distances  of  the  threads  in  the  two 
screws  is  contained  in  the  circle 
described  by  the  power. 

A  is  the  larger  screw,  B  W  the  smaller 
one.  C  D  is  the  lever  by  which  it  is  worked, 
and  E  F  the  stationary  nut.  The  pressure 
is  produced  at  W.  If  the  threads  of  the 
larger  screw  are  1  inch  apart,  and  those  of 
the  smaller  s/4  of  an  inch,  the  difference  is 
y4  of  an  inch.  Then,  if  the  extremities  of  the 
lever  describe  a  circle  of  100  inches,  the  ad- 
vantage will  be  equal  to  100  divided  by  J/4> 
or  400 ;  that  is,  a  power  of  1  pound  applied 


at  either  end  of  the  lever  will  produce  a  pressure  of  400  pounds  at  W. 


the  advantage  gained  by  this  machine  to  be  calculated  ?  264.  For  what  purposes  are 
screws  used  ?  265.  When  great  pressure  is  required,  what  difficulty  attends  the  use 
of  the  screw  ?  To  remedy  this,  what  ingenious  contrivance  is  used  ?  Describe  Hun- 
ter's Screw.  With  this  screw,  how  great  a  pressure  does  a  given  power  produce  f 


THE  ENDLESS   SCREW. 


117 


By  making  the  threads  of  the  two  screws  nearly  the  same  distance  apart, 
an  immense  power  is  obtained  without  diminishing  the  size  and  strength  of 
the  threads.  The  action  of  the  screw  is  of  course  proportionally  slow,  time 
being  always  lost  as  power  is  gained. 

266.  THE  ENDLESS  SCREW. — Instead  of  working  in  a 
nut,  a  screw  is  sometimes  made  to  act  on  teeth  cut  in  the 


circumference  of  a  wheel.  In  this 
case,  the  only  motion  of  the  screw 
is  round  its  axis.  The  winch  being 
turned,  the  threads  of  the  screw 
catch  the  teeth  of  the  wheel  and 
move  it  forward.  As  fast  as  one 
tooth  passes  out  of  reach,  another 
is  caught ;  and,  the  motion  being 
thus  continuous,  the  machine  is 
called  the  Endless  Screw.  Its  op- 
eration will  be  understood  from 


Fig.  125. 


Fig.   125,  where  it  is  combined 


THE  ENDLESS  8CEEW. 


with  a  wheel  and  axle  for  the  purpose  of  lifting  a  weight. 


EXAMPLES  FOR  PRACTICE. 

1.  (See  §204.)  A  lever  of  the  first  kind  is  20  inches  in  length :  the  long  arm 
is  15  inches ;  the  short  arm,  5.  How  great  a  power  will  balance  a  weight 
of  112  pounds  ?  With  the  same  lever,  how  great  a  weight  will  a  power 
of  50  pounds  balance  ? 

2;  A  former,  in  forcing  a  stump  from  the  ground,  uses  a  crow-bar  6  feet  long, 
which  he  rests  on  a  stone  five  feet  from  the  end  where  his  hand  is  ap- 
plied. The  resistance  of  the  stump  is  equal  to  a  weight  of  500  pounds ; 
how  great  a  pressure  must  he  exert,  to  move  it  ? 

3.  A  man  weighing  180  pounds,  and  a  boy  of  60  pounds,  are  teetering  on  a 

board  12  feet  long.    That  they  may  balance  each  other,  how  near  must 
the  man  sit  to  the  horse  on  which  the  board  rests  ? 

4.  A  man  whose  strength  enables  him  to  use  a  pressure  of  120  pounds,  wishes 

to  move  a  rock  weighing  600  pounds  with  a  lever  of  the  first  kind.   What 
must  be  the  comparative  length  of  the  arms  of  the  lever  ? 

If  with  his  unaided  strength  he  could  move  120  pounds  thirty  feet  in 
one  minute,  how  long  will  it  take  him  to  move  the  rock  with  the  lever 
the  same  distance  ? 

Illustrate  this  with  Fig.  124.  How  may  an  immense  power  be  gained  with  Hunter's 
Screw  ?  266.  Describe  the  Endless  Screw  and  its  mode  of  operation.  With  what  is 
it  combined  for  lifting  weights  ? 


118  MECHANICS. 

5.  (See  §  207.)  The  short  arm  of  a  steelyard  is  2  inches  long ;  at  its  end  a  10- 

pound  weight  is  suspended.  How  great  a  weight  must  be  attached  to 
the  other  end  to  balance  it,  the  length  of  the  steelyard  being  one  foot  ? 

6.  (See  §  213.)  There  is  a  compound  lever  formed  of  two  simple  ones,  the  first 

arms  of  which  are  10  inches  each,  and  the  short  arms  2  inches  each.  How 
great  a  weight  at  the  extremity  of  the  last  short  arm  will  be  supported 
by  a  power  of  1  pound  at  the  other  end  ? 

7.  (See  §  215.)  A  lever  of  the  second  kind  is  20  inches  long;  the  weight  is  5 

inches  from  the  fulcrum.  How  great  a  power  must  be  applied,  to  balance 
a  weight  of  112  pounds  ? 

8.  With  the  same  lever  as  in  the  last  sum,  how  great  a  weight  will  a  power 

of  50  pounds  balance  ? 

».  A  is  rowing  with  an  oar  9  feet  long,  and  has  his  row-lock  2  feet  from  his 
hand ;  B  rows  with  an  eight-foot  oar,  and  his  row-lock  is  1  foot  from  his 
hand.  If  they  strike  the  water  with  an  equal  length  of  oar,  which  ex- 
erts the  greater  power  on  the  boat  ? 

10.  (See  §  217.)  A  man  and  a  boy,  at  opposite  ends  of  a  bar  5  feet  long,  are 
carrying  a  150-pound  weight  suspended  between  them.     The  boy  can 
carry  but  30  pounds ;  how  far  from  his  end  must  the  weight  hang,  to 
give  him  that  portion  of  it,  and  the  man  the  rest  ? 

11.  Three  men  are  bearing  a  weight  suspended  from  a  bar  in  the  manner  shown 
in  Fig.  100.     The  single  man  at  one  end  is  twice  as  strong  as  each  of  the 
two  at  the  other  end.     How  must  the  weight  be  placed  (the  bar  being  4 
feet  long),  that  each  may  bear  a  part  proportioned  to  his  strength  ? 

12.  (See  §  219.)  A  lever  of  the  third  kind  is  20  inches  long ;  the  power  is  5 
inches  from  the  fulcrum.    How  great  must  it  be,  to  balance  a  weight  of 
112  pounds? 

13.  A  pair  of  pincers  is  6  inches  long.    How  great  a  force  must  be  applied, 
2  inches  from  the  top,  to  overcome  a  resistance  of  3  ounces  ? 

14.  The  distance  of  a  man's  hand  from  his  elbow  is  16  inches.    The  biceps 
muscle  is  inserted  in  his  fore-arm  2  inches  from  the  elbow.     With  how 
great  power  must  the  muscle  act  to  sustain  a  weight  of  56  pounds  in  the 
extended  hand  ? 

15.  (See  §220.)  The  circumference  of  a  wheel  ig  8  feet;  that  of  its  axle,  16 
inches.    The  weight,  including  friction,  is  60  pounds ;  how  great  a  pow- 
er will  be  required  to  raise  it  ? 

16.  The  pilot-wheel  of  a  boat  is  3  feet  in  diameter ;  the  axle  is  4  inches.    The 
resistance  of  the  rudder  is  180  pounds,  to  which  one-tenth  of  itself  must 
be  added  for  friction,  &c.    How  great  a  power  must  be  applied  to  the 
wheel,  to  move  the  rudder  ? 

17.  An  axle  one  foot  in  circumference,  fitted  with  a  winch  that  describes  a 
circle  of  6  feet,  is  used  for  drawing  water  from  a  well.   How  great  a  power 
will  it  take  to  move  60  pounds  of  water,  allowing  one-tenth  for  friction  ? 

18.  Four  men  are  drawing  in  an  anchor  that  weighs  1,000  pounds,  with  a 
capstan.    The  barrel  of  the  capstan  has  a  radius  of  6  inches.     The  circle 
described  by  the  handspikes  has  a  radius  of  5  feet.    How  great  a  pres- 
sure must  each  of  the  four  men  exert,  to  move  the  anchor? 


EXAMPLES  FOR  PRACTICE.  119 

19.  (See  §  232.)  With  a  fixed  pulley,  how  great  a  power  will  it  take  to  hoist 
a  weight  of  50  pounds,  20  percent.,  or  one-fifth,  being  added  for  friction  ? 

20.  (See  §  238.)  With  a  movable  pulley,  how  great  a  power  will  it  take  to 
hoist  a  weight  of  50  pounds,  twenty  per  cent,  being  allowed  for  friction  ? 

21.  (See  §  239.)  With  a  fixed  and  a  movable  pulley,  how  great  a  power  will 
it  take  to  hoist  a  weight  of  50  pounds,  40  per  cent.,  or  two-fifths,  being 
allowed  for  friction  ? 

22.  (See  §  241.)  With  two  fixed  and  two  movable  pulleys,  how  great  a  power 
will  it  take  to  hoist  a  weight  of  50  pounds,  60  per  cent.,  or  three-fifths, 
being  allowed  for  friction  ? 

23.  (See  §  242.)  How  great  a  power  will  it  take  to  hoist  a  weight  of  100  pounds 
with  one  of  White's  Pulleys  having  five  grooves  in  each  block,  35  per 
cent,  or  seven-twentieths,  being  allowed  for  friction  ? 

24.  (See  §  243.)  With  a  system  of  six  movable  pulleys,  having  each  its  own 
rope,  and  arranged  as  shown  in  Fig.  115,  how  great  a  weight  (including 
friction)  will  a  power  of  20  pounds  raise  ? 

25.  With  a  similar  system  of  five  movable  pulleys,  how  great  a  power  will  it 
take  to  balance  a  weight  of  64  pounds,  to  which  the  friction  of  the  pul- 
leys adds  50  per  cent.,  or  one-half  of  itself? — Ans.  3  pounds. 

[64  +  32  =  96  25  =  32  96-J-32  =  3  Answer.} 

26.  (See  §  247.)  How  great  a  power  will  be  required  to  balance  a  weight  of 
40  pounds  (friction  included),  on  an  inclined  plane,  whose  length  is  8 
times  its  height  ? 

27.  (See  §253.)  A  weight  of  1,500  pounds  is  to  be  raised  with  a  wedge  60 
inches  long  and  12  inches  high  at  its  head.  How  great  must  the  power  be  ? 

28.  A  builder  desires  to  raise  a  weight  of  900  pounds  with  two  similar  wedges, 
as  shown  in  Fig.  122.    Each  wedge  is  3  feet  long  and  9  inches  through 
at  the  head.    How  great  a  power  must  be  applied  to  each  ? 

29.  A  weight  of  1,020  pounds  is  to  be  lifted  l»/a  feet.    The  greatest  power 
that  can  be  applied  is  255  pounds.    Give  the  dimensions  of  the  wedge. 

30.  (See  §  257.)  Of  two  wedges  4  inches  thick  at  the  head  and  respectively  6 
and  8  inches  long,  which  can  be  driven  into  a  log  the  more  easily? 
Which  will  break  the  sooner,  both  being  made  of  the  same  material  ? 

31.  (See  §  262.)  How  great  a  pressure  (including  friction)  will  be  exerted  by 
a  power  of  15  pounds  applied  to  a  screw  whose  head  is  1  inch  in  circum- 
ference, and  whose  threads  are  one-eighth  of  an  inch  apart  ? 

32.  A  book-binder  has  a  press,  with  a  screw  whose  threads  are  one-third  of  an 
inch  apart,  and  a  nut  worked  by  a  lever  which  describes  a  circle  of  8  feet. 
How  great  a  pressure  will  a  power  of  5  pounds  applied  at  the  end  of  the 
lever  produce,  the  loss  by  friction  being  equivalent  to  240  pounds  ? 

33.  (See  §265.)  How  great  a  pressure  is  produced  by  a  power  of  1  pound 
with  one  of  Hunter's  Screws,  worked  by  a  lever  which  describes  a  circlo 
of  75  inches ;  the  threads  of  the  larger  screw  being  half  an  inch  apart 
and  those  of  the  smaller  one-third  of  an  inch,  33x/3  per  cent.,  or  one-third, 
of  the  pressure  being  deducted  for  friction  ? 


120  MECHANICS. 


CHAPTER  IX. 
MECHANICS   (CONTINUED). 

WHEELWORK. CLOCK   AND   WATCHWORK. 

267.  ALL  machines,  however  complicated,  are  combina- 
tions of  the  six  simple  mechanical  powers  described  in  the 
last  chapter.     The  chief  objects  in  combining  them  are  to 
gain  a  sufficient  degree  of  power,  and  to  give  such  a  direc- 
tion to  the  motion  as  will  make  the  machinery  do  the  work 
required. 

Whieelwork. 

268.  The  wheel  enters  more  largely  into  machinery  than 
any  other  of  the  Mechanical  Powers. 

269.  Several  wheels  combined  in  one  machine  are  called 
a  Train. 

270.  In  a  train  of  two  wheels,  the  one  that  imparts  the 
motion  is  called  the  Driver ;  the  one  that  receives  it,  the 
Follower. 

271.  MODES  OF  CONNECTION. — There  are  three  ways  in 
which  motion  may  be  transmitted  from  one  wheel  to  an- 
other:— 1.  By  the  friction  of  their  circumferences.     2.  By 
a  band.     3.  By  teeth  on  their  outer  rim. 

272.  Friction  of  the  Circumferences. — One  wheel  may 
move  another  by  rubbing  on  its  circumference,  or  outer 
rim.     The  wheels  are  so  placed  that  their  rims  touch,  and 
one  of  them  is  set  in  motion.     The  circumference  of  each 


26T.  Of  what  are  all  machines  combinations  ?  What  are  the  chief  objects  in  com- 
bining them  ?  268.  Which  of  the  mechanical  powers  enters  most  largely  into  ma- 
chinery? 269.  What  is  meant  by  a  Train  of  wheels?  270.  In  a  train  of  two  wheels, 
which  is  the  Driver ?  Which,  the  Follower?  271.  In  how  many  ways  may  motion 
be  transmitted  from  one  wheel  to  another  ?  Mention  them.  272.  How.  may  one 
wheel  be  made  to  move  another  by  rubbing  on  its  circumference  ?  What  is  the  ad- 


WHEEL WOEK. 


121 


Fig.  126. 


having  been  previously  roughened,  friction  prevents  the 
moving  wheel  from  slipping  over  the  one  at  rest,  and  mo- 
tion is  imparted  to  the  latter.  Wheels  thus  connected 
work  regularly  and  with  little  noise,  but  will  not  answer 
when  a  great  resistance  is  to  be  overcome,  and  hence  are 
not  much  used. 

273.  Sands. — One  wheel  may  be  made  to  move  an- 
other by  means  of  a  band  passed  round  both  circumfer- 
ences. Such  a  band  is  known  as  a  Wrapping  Connector. 
It  is  also  called  an  Endless  Band,  because,  its  ends  being 
joined,  we  never  seem  to  reach  them,  though  the  motion  is 
continuous  in  the  same  direction.  The  band  must  be 
stretched  so  tight  that  its  friction  on  the  wheels  may  be 
greater  than  the  resistance  to  be  overcome. 

Fig.  126  shows  how  wheels  are  connected  by  an 
endless  band.  If  the  follower  is  to  turn  in  the  same 
direction  as  the  driver,  the  band  is  passed  over  it 
without  crossing,  as  in  A ;  if  in  the  opposite  direction, 
the  band  is  crossed,  as  in  B. 

274.  The  bands  used  for  this  purpose  are  generally 
made  of  leather,  or  gutta  percha  [pert'-sha].     The 
wheels  may  be  far  apart,  if  necessary ;  and  on  this 
account,  as  well  as  because  a  great  amount  of  power 
may  thus  be  transmitted,  the  wrapping  connector  is 
much  used.    The  motion  imparted  is  exceedingly  reg- 
ular, any  little  inequalities  being  corrected  by  the 
stretching  of  the  band. 

275.  Fig.  127  shows  the  different  forms  given  to 
the  circumferences  of  wheels,  in  order  that  the  band 
may  not  slip  off.    A's  circumference  is  concave,  or 
hollows  towards  the  centre,  with  a  rim  on  each  side. 
B's  is  the  same,  with  a  row  of  pins  down  the  centre. 
C's  circumference  is  even  across,  with  a  rim  on  each 
side.    D  has  no  rim,  but  bulges  out  in  the  centre,  so 
that  when  the  band  tends  to  approach  one  side  it  is 
pulled  back  by  the  tightening  on  the  other. 

vantage,  and  what  the  disadvantage,  of  this  mode  of  connection  ?  273.  What  is  a 
"Wrapping  Connector  ?  What  other  name  is  given  to  it,  and  why  ?  How  tight  must 
the  band  be  ?  In  passing  from  the  driver  to  the  follower,  when  is  the  band  crossed, 
and  when  not  ?  274.  Of  what  are  endless  bands  usually  made  ?  By  what  advantages 
is  their  use  attended  ?  What  renders  the  motion  imparted  by  wrapping  connectors 
exceedingly  regular  ?  275.  Describe  the  different  forms  given  to  the  circumferences 
of  wheels  on  which  a  wrapping  connector  is  to  act.  276.  What  is  the  third  way  in 

6 


Fig.  127. 


122 


MECHANICS. 


Fig.  123.  276.    Teeth.— One  wheel  may  be  made  to 

move  another  by  means  of  teeth  on  the  circum- 
ference of  each.  A  toothed  wheel  is  shown  in 
Fig.  128. 

277.  Small  toothed  wheels  combined  with 
large  ones  are  called  Pinions,  and  their  teeth  Leaves. 

278.  Two  or  more  wheels  connected  by  teeth  are  called 
Gearing.     When  so  arranged  that  the  teeth  work  in  each 
other,  they  are  said  to  be  in  gear;   and  when  not,  out  of 
gear. 

Fig.  129.  Figure  129  shows  a 

train  of  wheels  and  pin- 
ions in  gear.  To  find 
how  great  a  weight  will 
be  balanced  by  a  given 
power  with  such  a 
train,  multiply  the  pow- 
er successively  by  the 
number  of  teeth  on  the 
wheels,  and  divide  by 
the  product  of  the  num- 
ber of  teeth  on  the  pin- 

TRAIN  OF  WHEELS  AND  PINIONS.  ions.    ;por  instance,  in 

Fig.  129,  let  the  first  large  wheel  have  18  teeth,  the  second  18,  the  third  27, 
and  the  fourth  27  ;  and  let  each  pinion  have  9  teeth.  Then  (leaving  friction 
out  of  account)  a  power  of  2  pounds  will  balance  a  weight  of  72  pounds.  For 

2  X  18  X 18  X  27  X  27  =  472392 

9X9X9X9  =  6561 
472392  divided  by  6561  =  72 

279.  KINDS  OF  TOOTHED  "WHEELS.  —  There  are  three 
kinds  of  toothed  wheels  ;  viz.,  Spur-wheels,  Crown-wheels, 
and  Bevel-wheels. 

280.  Spur-wheels. — Spur-wheels  have  their   teeth  per- 
pendicular to  their  axes,  as  shown  in  Fig.  129. 

The  teeth  are  either  made  in  one  piece  with  the  rim,  or 


which  one  wheel  may  be  made  to  move  another?  277.  What  are  Pinions?  What 
are  the  teeth  of  pinions  called  ?  278.  What  is  Gearing  ?  When  are  wheels  said  to  be 
in  gear  f  When  are  they  said  to  be  out  of  gear  f  What  does  Fig.  129  represent  ? 
With  such  a  train,  how  do  you  find  how  great  a  weight  will  be  balanced  by  a  given 
power?  Give  an  example.  279.  How  many  kinds  of  toothed  wheels  are  there! 
Name  them.  280.  Describe  Bpur-wheels.  How  are  the  teeth  made  ?  What 


WHEELWOEK. 


123 


consist  of  separate  pieces  set  into  the  rim. 
case,  they  are  called  Cogs. 

In  mills,  Cog-wheels  are  gen- 
erally used  with  Trundles,  or  Lan- 
terns, as  represented  in  Fig.  130. 

A  is  a  large  cog-wheel.  B  is  a  trundle, 
consisting  of  two  parallel  discs  and  an  inter- 
vening space  traversed  by  round  pins  called 
Staves,  so  arranged  as  to  receive  the  cogs  of 
the  other  wheel. 

Mill-wheels  are  generally  made  of  cast- 
iron  ;  but  they  are  found  to  work  most  smooth- 
ly when  one  of  them  has  wooden  instead  of 
iron  teeth.  Wooden  teeth  are  therefore  often 
set  in  the  larger  one,  which  is  then  called  a 
Mortice- wheel. 


In  the  latter 


COG-WHEEL  AND  TRUNDLE. 


281.    Crown-wheels.  —  Crown-wheels  have  their  teeth 
parallel  to  their  axes. 

Fig.  181. 

Fig.  132. 


CROWN-WHEEL  AND  PINION. 


HAND-MILL. 


Fig.  131  represents  the  contrate- wheel  and  pinion  of  a  watch.  B,  whose 
teeth  run  the  same  way  as  its  axis,  is  a  crown-wheel.  A,  whose  teeth  are  at 
right  angles  to  its  axis,  is  a  spur-wheel. 

Fig.  132  shows  how  a  crown-wheel  worked  by  a  winch  is  combined  with 
a  trundle  in  a  hand-mill  used  in  Germany  and  Northern  Europe.  The  crown- 
wheel moves  vertically,  but  it  communicates  a  horizontal  motion  to  the  trun- 
dle, which  in  turn  imparts  it  to  the  mill-stone. 

282.  Bevel-wheels. — Bevel-wheels  are  wheels  whose  teeth 


Cogs  ?  In  mills,  with  what  are  cog-wheels  generally  used  ?  Describe  a  Trundle 
Of  what  are  mill-wheels  generally  made  ?  What  is  said  of  their  Teeth  ?  What  is  a 
Mortice-wheel  ?  281.  Describe  Crown-wheels.  What  does  Fig.  131  represent  ?  De- 
scribe the  hand-mill  represented  in  Fig.  132.  282.  What  are  Bevel-wheels  ?  What 


124 


MECHANICS. 


form  any  other  angle  with 
their  axes  than  a  right  angle. 

A  pair  of  bevel-wheels 
in  gear  are  shown  in  Fig. 
133. 

283.    RACK  AND    PIN- 
ION.— Circular    motion    is 
converted  into  rectilinear 
(that  is,  motion  in  a  straight 
line)  by  means  of  the  rack         BEVEL-WHEELS. 
and  pinion,  represented  in  Fig.  134.     As 
the  pinion  A  revolves,  its  teeth  work  in 
Fig.  134.  those   of    the  rack 

B  C,  moving  it  for- 


Fig. 133. 


BACK.  ANI>  PIMION. 


Fig.  135. 


ward  in  a  straight  line. 

284.  FORGE-HAMMER. — A  toothed 
wheel  may  produce  an  alternate  up-and- 
down  motion,  as  in  the  case  of  the  Forge- 
hammer,  represented  in  Fig.  135. 

The  wheel  is  so  placed  that  its  teeth  successively 
come  in  contact  with  the  handle  of  the  hammer,  which 
turns  on  a  pivot.  As  the  wheel  revolves,  a  long 
tooth  carries  the  lower  end  of  the  handle  down  and 
raises  its  head.  As  soon  as  the  tooth  releases  the  handle,  the  head  of  the 
hammer  falls  on  the  anvil  by  its  own  weight.  A  new  tooth  then  comes  into 
play,  and  the  operation  is  repeated. 

285.  CRANKS. — The  Crank  is  much  used  in  machinery 
for  converting  circular  motion  into  rectilinear,  or  rectilinear 


THE  FORGE-HAMMEK. 


Fig.  136. 


into  circular.  It  has  different  forms,  but  is 
generally  made  by  bending  the  axle  in  the 
way  represented  in  Fig.  136.  As  the  wheel 
to  which  it  is  attached  turns,  the  crank  A 
also  revolves,  and  causes  the  rod  B,  with 
which  it  is  connected,  to  move  alternately 
up  and  down. 

does  Fig.  133  represent  ?  283.  How  may  circular  motion  be  converted  into  rectilin- 
ear? Describe  the  working  of  the  Rack  and  Pinion.  284.  What  kind  of  motion  does 
a  toothed  wheel  produce  in  the  case  of  the  forge-hammer  ?  Explain  the  working  of 
the  forge-hammer.  285.  For  what  is  the  Crank  used  ?  Describe  its  usual  form,  and 


THE  CRANK. 


THE   CRANK.  125 

The  point  at  which  the  rod  stands  at  right  angles  to  the 
axle  (as  in  the  Figure)  is  called  the  Dead-point.  Two  dead- 
points  occur  in  each  revolution.  When  at  either,  the  crank 
loses  its  power  for  the  instant ;  but  the  impetus  carries  it 
along,  and  as  soon  as  the  dead-point  is  passed  it  again  be- 
gins to  act. 

286.  Another  form  of  the  crank  is  exhib-  Fig.  137. 

ited  in  Fig.  137,  which  shows  how  a  wheel  is 
moved  by  a  treadle-board  worked  by  the  foot. 
A  is  the  treadle ;  B  C  is  a  cord  passed  round 
the  pulley  D,  and  attached  to  the  crank  E, 
which  is  connected  with  the  axle  of  the  wheel 
F.  When  the  foot  bears  the  treadle-board 
down,  the  end  of  the  crank  is  raised  to  its 
highest  point.  Here  it  would  remain  if  the 

f      ,  .  iV      t.       _J      t    A     j.t      *     j.  CKANK   AND   TREADLE. 

loot  were  kept  on  the  board ;  but,  the  toot 

being  removed,  the  impetus  of  the  wheel  carries  the  crank  round  again  to  its 
lowest  point,  raising  in  turn  the  end  of  the  treadle-board.  The  foot  is  now 
applied  again  with  the  same  effect  as  before,  and  continuous  motion  is  thus 
imparted  to  the  wheel. 

287.  FLY-WHEELS. — The  motion  of  machinery  must  be 
even  and  regular.  Both  power  and  resistance  must  there- 
fore act  uniformly;  if  either  increases  too  rapidly,  the  sud- 
den strain  is  apt  to  break  some  part  of  the  works.  To 
prevent  this,  the  fly-wheel  is  used. 

The  fly-wheel  appears  in  various  forms,  but  generally 
consists  of  a  heavy  iron  hoop  with  bars  meeting  in  the  cen- 
tre. It  is  set  in  motion  by  the  machinery,  and  by  reason 
of  its  weight  acquires  so  great  a  momentum  that  irregu- 
larities either  in  power  or  resistance,  unless  long  continued, 
have  but  little  effect.  If,  for  instance,  the  power  ceases  to 
act  for  a  moment,  or  the  resistance  suddenly  increases  or 
diminishes,  the  great  momentum  of  the  fly  prevents  the 
motion  of  the  machinery  from  varying  to  any  great  extent. 

288.  The  fly-wheel  also  accumulates  power,  and  thus  enables  a  machine 
to  overcome  a  greater  resistance  than  it  could  otherwise  do.  The  power, 

explain  its  operation.  What  is  meant  by  the  Dead-point  of  the  crank  ?  What  is  said 
of  the  crank  at  its  dead-point  ?  286.  What  does  Fig.  137  represent  ?  Explain  the  op- 
eration of  the  crank  and  treadle.  287.  For  what  is  the  Fly-wheel  used  ?  Of  what 
does  it  generally  consist  ?  Explain  how  the  fly-wheel  prevents  irregularities  of  mo- 
tion. 288.  For  what  other  purpose  is  the  fly-wheel  used?  How  does  the  fly-wheel 


120  MECHANICS. 

allowed  to  act  on  the  fly  alone  for  a  short  time,  gives  it  an  immense  momen- 
tum ;  and  this  momentum  directly  aids  the  power,  when  the  machine  is  ap- 
plied to  the  required  work. 

Clock  and  Watch  Work. 

289.  One  of  the  commonest  and  most  ingenious  appli- 
cations of  wheel  work  is  exhibited  in  clocks  and  watches. 

290.  HISTORY. — The  advantages  of  combining  wheels' 
and  pinions  were  partially  known  as  far  back  as  the  time 
of  Archimedes;  yet  they  were  comparatively  little  used  in 
machinery,  and  not  at  all  for  the  measurement  of  time. 

Instead  of  clocks  and  watches,  consisting  of  trains  of  wheels,  the  ancients 
used  the  sun-dial,  and  clep'-sy-dra  or  water-clock.  The  former  indicated  the 
hour  by  the  position  of  the  shadow  cast  by  a  style,  or  pin,  on  a  metallic  plate ; 
the  latter,  by  the  flow  of  water  from  a  vessel  with  a  small  hole  in  the  bottom. 
The  dial  was  of  course  useless  at  night ;  and  neither  it  nor  the  clepsydra, 
however  carefully  regulated,  could  measure  time  with  apy  great  degree  of 
accuracy. 

Even  Alfred  the  Great,  985  years  after  Christ,  had  no  suitable  instrument 
for  measuring  time.  To  tell  the  passing  hours,  he  used  wax  candles  twelve 
inches  long  and  of  uniform  thickness,  six  of  which  lasted  about  a  day.  Marks 
on  the  surface  at  equal  intervals  denoted  hours  and  their  subdivisions,  each 
inch  of  candle  that  burned  showing  that  about  twenty  minutes  had  passed. 
To  prevent  currents  of  air  from  making  his  candles  burn  irregularly,  he  en- 
closed them  in  cases  of  thin,  transparent  horn, — and  hence  the  origin  of  th« 
lantern. 

291.  Clocks  moved  by  weights  were  known  to  the  Sar- 
acens as  early  as  the  eleventh  century.     The  first  made  in 
England  (about  1288  A.  D.)  was  considered  so  great  a  work 
that  a  high  dignitary  was  appointed  to  take  care  of  it,  and 
paid  for  so  doing  from  the  public  treasury.     The  usefulness 
of  clocks  was  greatly  increased  by  the  application  of  the 
pendulum,  which  was  made  about  the  middle  of  the  seven- 
teenth century. 

Watches  seem  to  have  been  first  made  in  the  six- 
aid  the  power?  289.  In  what  do  we  find  one  of  the  most  ingenious  applications  of 
wheel-work  ?  290.  What  is  said  of  the  knowledge  of  wheel-work  possessed  by  the 
ancients?  "What  did  the  ancients  use  for  the  measurement  of  time?  How  did  the 
sun-dial  indicate  the  hour  ?  How,  the  clepsydra  ?  What  is  said  of  the  accuracy  of 
these  instruments  ?  How  did  Alfred  the  Great  measure  time  ?  What  was  the  origin 
of  the  lantern  ?  291.  "When  were  clocks  moved  by  weights  first  made  by  the  Sara- 
eens?  When  was  tho  first  made  in  England  ?  How  was  this  clock  regarded  ?  What 


CLOCK  AND   WATCH   WOKK.  127 

teenth  century,  though  it  is  not  known  who  was  their  in- 
ventor. For  a  time  they  were  quite  imperfect,  requiring  to 
be  wound  twice  a  day,  and  having  neither  second  nor  min- 
ute hand.  The  addition  of  the  hair-spring  to  the  balance, 
by  Dr.  Hooke,  in  1658,  was  the  first  great  improvement. 
Others  have  since  been  devised ;  and  chronometers  (as  the 
best  watches,  manufactured  for  astronomers  and  naviga- 
tors, are  called)  are  now  made  so  perfect  as  not  to  deviate 
a  minute  in  six  months,  even  when  exposed  to  great  varia- 
tions of  temperature. 

292.  CLOCK-WORK. — In  clocks,  except  such  as  are  moved 
by  springs  similarly  to  watches,  the  moving  power  is  a 
weight ;  to  which,  when  wound  up,  gravity  gives  a  constant 
downward  tendency.     In  its  effort  to  descend,  it  sets  in 
motion  a  train  of  wheels  and  pinions ;  and  they  move  the 
hands  which  indicate  the  hours  and  minutes  on  the  face. 

The  motion  of  the  wheels,  though  caused  by 
the  weight,  is  regulated  by  the  pendulum  and  an 
apparatus  called  the  Escapement,  shown  in  Fig. 
138.  The  crutch  ABC  moves  with  the  pendu- 
lum. As  the  latter  vibrates,  the  pallets  B,  C,  are 
alternately  raised  far  enough  to  let  one  tooth  of 
the  scape-wheel  pass,  its  motion  at  other  times 
being  checked  by  the  entrance  of  one  of  the 
pallets  between  the  teeth.  Hence,  though  the 
weight  is  wound  up,  the  clock  does  not  go  till 
the  pendulum  is  set  in  motion.  If  the  pendu- 
lum and  escapement  are  removed,  the  weight 
runs  down  unchecked,  turning  the  various  wheels  TIIE  ESCAPEMENT. 

with  great  rapidity.  The  motion  of  the  wheels  is  thus  made  uniform  by  the 
pendulum ;  and  by  shortening  or  lengthening  it  we  can  make  the  clock  go 
faster  or  slower. 

293.  WATCH-WORK. — In  a  watch,  there  is  no  room  for 
a  weight  or  pendulum ;  hence  a  spring,  called  the  main- 


greatly  increased  the  usefulness  of  clocks  ?  When  were  watches  first  made  ?  What 
was  the  character  of  those  first  constructed  ?  What  was  the  first  great  improve- 
ment ?  What  is  said  of  the  chronometers  made  at  the  present  day  ?  292.  What  is  the 
moving  power  in  clocks  ?  How  does  the  weight  set  the  clock  in  motion  ?  How  ia 
the  motion  of  the  wheels  regulated  ?  Explain,  with  Fig.  138,  how  the  Escapement 
regulates  the  motion.  If  the  pendulum  and  escapement  are  removed,  what  is  tho 
consequence  ?  How  is  tho  clock  made  to  go  iaster  or  slower  ?  293.  In  a  watch,  what 


128  MECHANICS. 

spring,  is  substituted  for  the  former  as  a  moving  power, 
while  the  balance  and  hair-spring  take  the  place  of  the  lat- 
ter as  a  regulator. 

The  main-spring  is  either  fixed  to  an  axle  capable  of  revolving,  as  shown 
at  0  P  in  Fig.  140,  or  is  contained  within  a  hollow  barrel,  connected  by  a  chain 
with  a  conical  axle,  called  the  fusee,  represented  in  Fig.  139.  A  is  the  barrel, 
Fig.  139.  within  which  and  out  of  sight  is  the 

main-spring,  having  one  end  attached  to 
the  inner  surface  of  the  barrel,  and  the 
other  fastened  to  a  fixed  axle  passing 
through  the  barrel.  B  is  the  fusee. 

The  watch  is  wound  up  with  a  key, 
applied  to  the  square  projecting  from 

the  fusee.  By  turning  the  square  the  chain  is  drawn  off  from  the  barrel  and 
wound  round  the  fusee.  The  barrel  is  thus  turned  till  the  spring  in  the  in- 
side is  tightly  coiled.  This  spring,  by  reason  of  its  elasticity,  tends  to  un- 
coil, and  in  so  doing  moves  the  barrel  round,  drawing  off  the  chain  from  the 
fusee,  and  winding  it  again  around  the  barrel.  The  fusee  is  thus  turned,  and 
carries  with  it  the  first  wheel  of  the  train,  which  imparts  motion  to  all  the 
rest.  When  the  spring  has  uncoiled  itself,  the  chain,  being  entirely  wound 
round  the  barrel,  ceases  to  move  the  fusee,  and  all  the  wheels  come  to  rest. 
The  watch  is  then  said  to  run  down. 

The  reason  of  the  peculiar  shape  of  the  fusee  is  this.  The  power  of  the 
spring  is  proportioned  to  the  tightness  with  which  it  is  coiled,  and  hence  is 
greatest  when  the  watch  is  first  wound.  The  chain  is  consequently  then 
made  to  act  on  the  smallest  part  of  the  fusee ;  because,  the  nearer  to  the  axis 
the  force  is  applied,  the  less  its  power  of  producing  motion.  As  the  spring 
gradually  uncoils,  its  power  is  weakened  and  it  is  made  to  act  on  a  larger 
part  of  the  fusee.  By  thus  adjusting  the  size  of  the  fusee  to  the  varying 
power  of  the  spring,  a  uniform  effect  is  secured. 

294.  An  escapement  similar  to  that  used  in  clocks  connects  the  moving 
power  with  the  balance.  To  the  latter,  also,  a  very  fine  spiral  spring  is  at- 
tached, which  is  fastened  at  its  other  end  to  a  fixed  support.  The  watch  is 
regulated  by  shortening  or  lengthening  this  spring,  the  balance  being  made 
to  vibrate  faster  or  slower  accordingly. 

295.  The  works  of  an  ordinary  watch  are  shown  in  Fig. 
140.  For  convenience  of  inspection,  they  are  arranged  in 
a  line,  and  the  distance  between  the  two  plates,  and  also 
between  the  upper  plate  and  the  face,  is  increased. 

takes  the  place  of  the  weight,  and  what  of  the  pendulum  ?  What  two  ways  are  there 
of  fixing  the  main-spring?  Explain  Fig.  139.  How  is  the  watch  wound  up  ?  Ex. 
plain  the  working  of  the  fusee.  When  does  the  watch  run  down,  and  why  does  motion 
then  cease  1  What  is  the  reason  of  the  peculiar  shape  of  the  fusee  ?  294.  What  con- 
nects the  moving  power  with  the  balance  ?  What  is  attached  to  the  balance  ?  How 


Fig.  140. 


WORKS  OF  A  WATCH. 


WATCH-WORK.    ^       ^ 

0  P  is  the  matntsprJna,  attacaecr; 
to  its  axle,  without  a  lu^f  V*§he  un- 
coiling of  the  spring  carries  tn^ax^e 
round,  and  with  it  the  great  wh6ei  1f.  • 
N  works  in  the  pinion  a,  and  by  turn- 
ing it  turns  also  the  centre-wheel  M  on 
the  same  axis,  so  called  from  being  in 
the  centre  of  the 
watch.    M  turns 
the  pinion  b  and 
the  third   wheel 
L,  which  in  turn 
works  in  the  pin- 
ion c  and  causes 
the  second  or  con- 
trate-wheel  R,  on 
the   same   axis, 

to  revolve.     R  works  in  the  pinion  d  and  carries  round  the  balance  or  crown 
wheel  C,  which  is  on  the  same  axis  with  it. 

The  saw-like  teeth  of  the  balance-wheel  are  checked  (as  in  the  case  of  the 
escapement  of  a  clock)  by  the  pallets  p,p,  which  are  projecting  pins  on  the 
verge  of  the  balance  A.  The  hair-spring,  fastened  at  one  end  to  a  fixed  sup- 
port, and  at  the  other  to  the  balance,  may  be  shortened  by  the  curb  or  reg- 
ulator, if  the  watch  goes  too  slow,  or  lengthened  if  it  goes  too  fast,  thus  con- 
trolling the  motion  of  the  balance  and  consequently  that  of  the  other  wheels. 
296.  The  force  of  the  main-spring  is  so  adjusted  as  to  make  the  great 
wheel  N  revolve  once  in  four  hours.  The  spring  generally  turns  it  seven  or 
eight  times  round  before  it  is  uncoiled,  so  that  with  one  winding  the  watch 
runs  twenty-eight  or  thirty-two  hours.  The  great  wheel  N  has  forty-eight 
teeth,  the  pinion  a  but  twelve ;  so  that  a  and  the  centre-wheel  M  revolve  once 
every  hour,  and  their  axle,  carried  through  to  the  face,  bears  the  minute- 
hand. 

Between  the  face  and  the  upper  plate  is  a  train  of  pinions  and  wheels  con- 
nected with  the  axle  of  the  centre-wheel.  They  are  so  adjusted  that  the  wheel 
V"  revolves  once  in  twelve  hours.  V  carries  the  hour-hand.  It  is  attached 
to  a  hollow  axle,  through  which  the  axle  of  the  centre-wheel  passes  to  carry 
the  minute-hand. 

297.  Thus  we  see  that  the  works  of  a  watch  are  nothing 
more  than  an  ingenious  combination  of  wheels,  moved  by  a 
spring  and  regulated  by  a  balance.  The  arrangement  of  the 

Is  the  watch  regulated?  295.  "What  does  Fig.  140  represent?  With  the  aid  of  Fig. 
140,  describe  the  works  of  a  watch  and  their  mode  of  operation.  How  is  the  watch 
regulated?  296.  How  great  a  force  is  generally  given  to  the  main-spring?  How 
long  does  the  watch  run  with  one  winding  ?  Explain  the  arrangement  of  the  minute- 
hand.  Explain  that  of  the  hour-hand.  297.  Of  what,  as  we  have  seen,  do  the  works 
6* 


130  HYDROSTATICS. 

wheels  and  pinions  is  such,  that  there  is  a  constant  increase 
of  velocity  and  a  corresponding  loss  of  power.  The  great 
wheel,  which  begins  the  train,  re  v-olves  once  in  four  hours  ; 
the  balance,  which  closes  it,  revolves  in  one-fifth  of  a  sec- 
ond ;  but  the  force  of  the  spring  becomes  so  attenuated 
by  the  tune  it  reaches  the  balance,  that  the  slightest  addi- 
tional resistance  there,  a  particle  of  dust  or  even  a  thicken- 
ing of  the  oil  used  to  prevent  friction,  deranges,  and  may 
stop,  the  action  of  the  whole. 


CHAPTER    X. 
MECHANICS    (CONTINUED). 

HYDROSTATICS. 

298.  HYDROSTATICS  and  Hydraulics  are  branches  of  Me- 
chanics that  treat  of  liquids. 

Hydrostatics  is  the  science  that  treats  of  liquids  at  rest. 
Hydraulics  is  the  science  that  treats  of  liquids  in  mo- 
tion, and  the  machines  in  which  they  are  applied. 

299.  The  principles  of  Hydrostatics  and  Hydraulics  are 
equally  true  of  all  liquids  ;  but  it  is  in  water,  which  is  the 
commonest  liquid,  that  we  most  frequently  see  them  ex- 
hibited. 

Water  abounds  on  the  earth's  surface.  It  covers  more  than  two-thirds  of 
the  globe,  and  constitutes  three-fourths  of  the  substance  of  plants  and  ani- 
mals. 

300.  NATURE  OF  LIQUIDS. — Liquids  differ  from  solids  in 
having  but  little  cohesion. 

of  a  watch  consist?  What  is  said  of  the  arrangement  of  the  wheels  and  pinions? 
What  is  the  comparative  velocity  of  the  great  wheel  and  the  balance  ?  What  is  said 
of  the  force  of  the  spring  hy  the  time  it  reaches  the  balance  ? 

298.  What  sciences  treat  of  liquids  ?  What  is  Hydrostatics  ?  What  is  Hydraulics  ? 
299.  What  is  said  of  the  principles  of  hydrostatics  and  hydraulics  ?  How  much  of 
the  globe  is  covered  with  water  ?  How  much  of  the  substance  of  plants  and  animaft 
Consists  of  water  ?  300.  In  what  respect  do  liquids  differ  from  solids  ?  What  shows 


NATURE   OP   LIQUIDS.  .      131 

Cohesion  is  not  entirely  wanting  in  liquids,  as  is  proved  by  their  parti- 
cles' forming  in  drops ;  but  it  is  so  weak  as  to  be  easily  overcome.  Thick 
and  sticky  liquids,  like  oil  and  molasses,  have  a  greater  degree  of  cohesion 
than  thin  ones,  like  water  and  alcohol. 

301.  Liquids  were  long  thought  to  be  incompressible, 
but  experiment  has  proved  the  reverse.     Submitted  to  a 
pressure  of  15,000  pounds  to  the  square  inch,  a  liquid  loses 
one-twenty-fourth  of  its  bulk.     Were  the  ocean  at  any  point 
a  hundred  miles  deep,  the  pressure  of  the  water  above  on 
that  at  the  bottom  would  reduce  it  to  less  than  half  its 
proper  volume. 

302.  To  distinguish  them  from  the  gases,  liquids  are 
often  called  non-elastic  fluids ;  yet  they  are  not  devoid  of 
elasticity. 

To  prove  this,  after  compressing  a  body  of  water,  remove  the  pressure, 
and  it  will  resume  its  former  bulk.  Again,  if  a  knife-blade  be  brought  in 
contact  with  a  drop  of  water  hanging  from  a  surface,  the  drop  may  be  elon- 
gated by  slowly  drawing  away  the  blade  ;  but  it  immediately  returns  to  its 
original  shape,  if  the  blade  is  entirely  removed  without  detaching  the  drop 
from  the  surface. 

Law  of  Hydrostatics. 

303.  Water  at  rest  always  finds  its  level. 

No  matter  what  the  size  or  shape  of  a  body  of  water  may  be,  its  surface 
Las  the  same  level  throughout ;  that  is,  it  is  equally  distant  at  every  point  from 
the  earth's  centre.  Accordingly,  the  surface  of  the  ocean  is  spherical ;  and 
this  we  know  to  be  the  case  from  always  seeing  the  mast  of  a  vessel  approach- 
ing in  the  distance  before  we  see  the  hull.  In  small  masses  of  liquids,  no 
convexity  is  perceptible ;  and  we  may  consider  their  surfaces  as  perfectly  flat. 

304.  The  tea-pot  affords  us  a  familiar  illustration  of  this  law.  The  tea 
always  rises  as  high  in  the  spout  as  in  the  body  of  the  pot ;  and,  if  the  body 
is  higher  than  the  spout,  it  will  pour  out  from  the  latter  when  the  pot  is 
filled. 

So,  let  there  be  a  number  of  vessels  having  communication  at  their  bases, 
as  shown  in  Fig.  141.  If  water  be  poured  into  any  of  them,  it  will  rise  to 

that  cohesion  is  not  entirely  wanting  in  liquids?  What  liquids  have  the  most  cohe- 
sion ?  301.  What,  is  said  respecting  the  compressibility  of  liquids  ?  If  the  ocean  were 
a  hundred  miles  deep,  what  would  be  the  consequence  of  the  pressure  ?  302.  What 
are  liquids  often  called,  to  distinguish  them  from  gases?  Is  the  name  strictly  correct? 
Prove  that  liquids  are  elastic  ?  303.  What  is  the  great  law  of  Hydrostatics  ?  What 
do  we  mean,  when  we  say  that  a  body  of  water  has  the  same  level  throughout  ?  What 
Hort  of  a  surface  must  the  ocean  have?  What  evidence  is  there  of  this?  How  may 
we  regard  the  surfaces  of  small  bodies  of  liquids  ?  304.  Show  how  the  tea-pot  illus- 


132 


HYDEOSTATICS. 


Fig.  141.  the  same  level  in  all, 

no  matter  how  they 
may  differ  in  shape  ot 
size.  In  like  manner, 
if  there  be  subterra- 
neous connection  be- 
tween a  river  affected 
by  the  tide  and  pools 
near  its  banks,  the  wa- 
ter in  the  pools  will 
rise  and  fall  simulta- 
neously with  that  in  the  river. 

305.  We  take  advantage  of  this  law  in  supplying  cities 
with  water  from  elevated  ponds  or  streams.  The  water 
may  be  conveyed  in  pipes  any  distance,  may  be  carried  be- 
neath deep  ravines  or  the  beds  of  rivers,  and  when  released 
from  the  pipe  at  any  point  will  rise  to  the  level  from  which 
it  started. 

Fig.  142.  ;  .: 


Thus,  in  Fig.  142,  the  pond  A  is  made  to  supply  the  house  D  with  water 
by  means  of  pipes  carried  down  into  the  valley,  under  the  stream  B  and  over 
the  bridge  C.  In  the  house  it  will  reach  the  level  of  the  pond  from  which  it 
was  taken,  shown  by  the  dotted  line. 

Fountains  formed  by  tapping  the  pipe  at  any  point,  rise,  theoretically,  to 
the  same  level,  as  seen  in  the  plate,  but  are  prevented  from  quite  reaching 
it  by  the  resistance  of  the  air  and  the  check  which  the  ascending  stream  re- 
ceives from  the  falling  drops. 

306.  The  ancient  Romans  appear  to  have  known  that  water  conducted 
in  pipes  will  find  its  level ;  yet  so  difficult  did  they  find  it  to  make  water- 


trates  this  law.  Illustrate  it  with  Fig.  141.  How  does  this  law  apply  in  the  case  of 
pools  connected  with  tide-water?  305.  To  what  practical  purpose  is  this  principle 
applied  ?  Illustrate  this  with  Fig.  142.  How  high  will  fountains  formed  by  tapping 
the  pipe  rise?  806.  How  did  the  ancient  Komans  convey  their  supplies  of  water? 


ARTESIAN  WELLS.  133 

tight  joints,  that,  instead  of  employing  pipes,  they  conveyed  their  water 
through  vast  level  aqueducts,  bridging  at  an  immense  expense  such  ravines 
and  valleys  as  lay  in  their  course.  In  modern  times,  iron  pipes  laid  beneath 
the  surface,  however  much  it  may  be  depressed,  accomplish  the  same  object 
with  much  less  cost,  the  water  always  rising  to  its  original  level  when  al- 
lowed to  do  so.  The  lower  the  pipes  are  sunk,  the  stronger  they  should  be ; 
for  the  upward  pressure  of  the  water,  tending  to  resume  its  level,  increases 
in  proportion  to  the  depth. 

307.  Artesian  Wells. — It  is  on  this  principle,  also,  that 
Artesian  Wells  are  made.     They  are  so  called  from  the 
province  of  Artois  [ahr-twah'],  in  France,  the  first  district 
of  Europe  where  they  were  extensively  introduced,  though 
known  to  the  Chinese  for  centuries. 

The  outer  crust  of  the  earth  consists  of  different  strata,  or  layers ;  some 
of  which  (rock  and  clay,  for  instance)  are  impervious  to  water,  and  others 
not  (such  as  gravel  and  chalk).  If  a  stratum  which  allows  water  to  flow 
through  it  is  enclosed,  after  leaving  the  surface,  between  two  impervious 
layers,  and  thus  descends  to  a  lower  level,  the  water  received  by  this  stratum 
at  the  surface,  unable  to  pass  out  above  or  below,  collects  in  it  throughout  its 
whole  length.  Let  an  opening  then  be  made  at  any  point  into  this  reservoir 
through  the  impervious  stratum  above,  and  the  water  will  at  once  rise  to 
find  its  level. 

Such  openings  are  Artesian  wells.  They  have  been  carried  in  some  cases 
a  third  of  a  mile  below  the  surface  ;  and  so  abundant  is  their  supply  of  water 
that  a  single  well  of  this  kind  at  Paris  has  been  computed  to  yield  14,000,000 
gallons  daily.  The  elevated  end  may  be  several  hundred  miles  distant ;  it 
matters  not  how  far.  It  is  thought  that  the  deserts  of  Arabia  and  Africa 
might  be  supplied  with  water,  and  thus  rendered  habitable,  by  means  of  Ar- 
tesian wells. 

308.  /Springs. — Springs  have  a  similar  origin.     The  rain 
drunk  up  by  the  earth's   surface  gradually  sinks,  till  it 
reaches  an  impervious  stratum.     Along  this  it  runs,  receiv- 
ing additions  as  it  goes,  till  it  finds  vent  in  some  natural 
opening. 

In  ordinary  wells,  the  water  does  not  rise  to  the  earth's  surface,  because 
it  does  not  come  from  an  elevated  stratum. 


Why  did  they  not  employ  pipes  ?  What  precaution  must  be  taken,  in  consequence 
of  the  upward  pressure  of  the  water?  307.  What  wells  are  made  on  this  principle  ? 
Why  are  Artesian  Wells  so  called  ?  Explain  their  working.  How  low  have  they 
been  carried  ?  How  much  water  does  the  well  at  Paris  supply  ?  How  far  off  may 
the  elevated  end  of  the  stratum  be  ?  What  is  thought  respecting  the  deserts  of  Ara- 
bia and  Africa?  308.  Explain  the  origin  of  springs.  Why  does  not  the  water  rise  in 


134  HYDROSTATICS. 

309.  Locks. — We  are  enabled  to  run  canals  through  un- 
even tracts  by  taking  advantage  of  the  fact  that  water  al- 
ways finds  its  level.  If  the  bottom  of  the  canal  were  not  of 
a  uniform  grade,  the  water  would  run  towards  the  lower  end 
and  inundate  the  surrounding  country.  When,  therefore, 
the  ground  is  uneven,  the  canal  is  built  in  sections,  each  level 
in  itself,  but  of  a  different  grade  from  the  one  next  to  ify 
with  which  it  is  connected  by  a  compartment  called  a  Lock. 

Fig.  143. 
E 


Let  AB  represent  a  canal,  the  upper  section  of  which,  A,  is  fifteen  feet 
higher  than  the  lower  section  B.  A  boat  is  passed  from  one  to  the  other  by 
means  of  the  lock  C,  which  communicates  with  either  section,  as  may  be  de- 
sired, by  opening  sliding  valves  in  the  lock-gates  D,  E.  When  a  boat  is  going 
down,  the  gate  E  is  closed  and  D  is  opened  till  the  water  in  the  lock  assumes 
the  same  level  as  in  A.  The  boat  is  then  brought  into  the  lock ;  the  gate  D  is 
closed  and  E  is  opened.  The  water,  gradually  sinking  in  the  lock,  bears  the 
boat  along  with  it  till  it  reaches  the  same  level  as  in  B.  In  going  up,  the  op- 
eration is  reversed.  The  boat  having  passed  from  B  into  the  lock,  E  is  closed 
and  D  opened.  The  water  rushes  in  to  find  its  level,  and  the  boat  is  raised 
till  it  stands  at  the  same  height  as  the  water  in  A. 

310.  The  Spirit  Level. — The   Spirit   Level,  an  instru- 
ment much  used  by  surveyors,  masons,  and  others,  operates 
Fig.  144  on  *n*s  sams   principle.     It  consists 

of  a  glass  tube  (see  Fig.  144)  near- 
ly filled  with  colored  alcohol,  just 
enough  air  being  allowed  to  remain 
in  it  to  form  a  bubble.  The  tube  is  then  closed,  and  fixed 
in  a  wooden  or  metallic  case. 

On  being  applied  to  a  surface,  if  the  latter  is  perfectly  level,  the  air-bub- 
ble will  rest  midway  of  the  tube,  in  its  highest  point  which  has  been  found 

Ordinary  wells  ?    309.  How  are  we  enabled  to  ran  canals  through  uneven  tracts  of 
country  ?    With  the  aid  of  Fig.  143,  show  the  workings  of  a  Lock.    310.  What  is  the 


THE  SPIRIT   LEVEL* 


PKESSUKE   OF  LIQUIDS. 


135 


by  previous  experiment  and  marked.  If  the  bubble  rests  in  any  other  place, 
it  shows  that  one  end  of  the  tube  is  higher  than  the  other,  and  consequently 
that  the  surface  on  which  it  rests  is  not  level. 

The  tube  is  sometimes  made  of  a  different  form,  and  nearly  filled  with  wa- 
ter instead  of  alcohol ;  the  instrument  is  then  known  as  the  Water  Level. 


Fig.  145. 


Pressure  of  Liquids. 

311.  FIKST  LAW. — Liquids,  subjected  to  pressure,  trans- 
mit it  undiminished  in  all  directions. 

Solids  transmit  pressure  only  in  the  line  in  which  it  is 
exerted;  liquids  transmit  it  in  every  direction.  This  is 
proved  by  experiment. 

In  Fig.  145,  A  represents  a  glass  vessel  of  water,  to  the 
neck  of  which  a  piston,  B,  is  tightly  fitted.  Tubes  are 
inserted  at  intervals  through  orifices  in  the  sides.  As  the 
piston  is  driven  down,  the  pressure  is  felt  alike  at  all  points 
of  the  vessel,  as  is  shown  by  the  flow  of  the  water  from 
the  tubes. 


312.  SECOND  LAW. — Liquids,  influenced 
~by  gravity  alone,  press  in  all  directions. 

Bore  a  hole  in  the  bottom  of  a  pail  filled  with  water ; 
the  water  rushes  out — this  proves  its  downward  pressure. 

Bore  a  hole  in  the  side  of  the  same  pail :  the  water 
rushes  out — this  proves  its  lateral  pressure. 

Bore  a  hole  in  the  bottom  of  a  boat ;  the  water  rushes 
in — this  shows  its  upward  pressure. 

313.  THIKD  LAW. — The  pressure  of  liquids  in  every  di- 
rection is  proportioned  to  their  depth. 

The  downward  pressure  of  liquids  increases  with  their  depth.  To  prove 
this,  take  four  tubes  of  equal  diameter,  and  over  one  end  of  each  tie  a  piece 
of  very  thin  india  rubber.  Fill  them  with  water  to  different  heights,  say  5, 
10,  20,  and  30  inches.  The  india  rubber  will  be  distended  the  most  in  the 
one  containing  the  greatest  depth  of  liquid. 

The  lateral  pressure  of  liquids  increases  with  their  depth.    Hence  dams 


Spirit  Level  ?  Of  what  does  it  consist?  How  is  the  spirit  level  used?  What  is  the 
Water  Level  ?  311.  What  is  the  first  law  relating  to  the  pressure  of  liquids  ?  What  is 
the  difference  between  solids  and  liquids  in  this  respect?  Illustrate  this  law  with 
Fig.  145.  312.  What  is  the  second  law  relating  to  the  pressure  of  liquids  ?  Prove  the 
downward  pressure  of  liquids.  Prove  their  lateral  pressure.  Prove  their  upward  pres- 
sure. 313.  What  is  the  third  law  relating  to  the  pressure  of  liquids  ?  What  experf- 
fiaent  proves  that  the  downward  pressure  of  liquids  is  proportioned  to  their  depth? 


136 


HYDROSTATICS. 


Fig.  146. 


and  sea-walls  should  increase  in  strength  towards  their  bases.  On  the  same 
principle,  barrels  holding  liquids  should  be  more  securely  hooped  at  bottom 
than  at  top. 

The  upward  pressure  of  liquids  increases  with  their 
depth.  This  is  shown  by  the  experiment  represented  in 
Fig.  146.  A  B  is  an  open  tube,  ground  perfectly  smooth  on 
the  lower  end.  C  is  a  plate  of  lead  attached  to  a  string. 
Pass  the  string  through  the  tube,  and  with  it  keep  the  lead 
plate  close  against  the  ground  end ;  then  introduce  the 
whole  into  a  deep  vessel  of  water.  When  it  has  descended 
an  inch  or  two,  let  go  the  string,  and  the  lead  will  sink.  Let 
it  go  near  the  bottom  of  the  vessel,  and,  as  shown  in  the 
Figure,  the  lead  will  be  supported  by  the  water.  The  up- 
ward pressure  has  therefore  increased  with  the  depth. 

314.  At  great  depths  the  pressure  of  water  becomes  im- 
mense ;  neither  divers  nor  fish  can  endure  it.    Strong  glass 
bottles,  empty  and  tightly  corked,  are  often  let  down  with  cords  at  sea,  and 
the  pressure  is  generally  sufficient  to  break  them  at  a  depth  of  60  feet.    If 
the  bottle  does  not  break,  either  the  cork  is  driven  in  or  water 
Fig.^147.      enters  through  its  pores.     The  hardest  wood,  sunk  to  a  great 
depth,  has  its  pores  so  thoroughly  filled  with  wa'ter  as  to  become 
incapable  of  rising.    Hence,  when  a  ship  goes  down  at  sea,  her 
timbers  are  never  seen  again. 

3 15., This  law  leads  to  wonderful  results.  Ef- 
fects almost  incredible  may  be  produced  by  an  in- 
significant body  of  liquid  so  disposed  as  to  have 
considerable  depth. 

We  may,  for  example,  burst  a  stout  cask  with  a  few  ounces  of 
water.  Having  filled  the  cask  with  water  and  inserted  in  its  top 
a  long  tube  communicating  with  the  inside,  we  may  force  the 
staves  asunder,  however  tightly  hooped,  by  simply  pouring  wa- 
ter into  the  tube. 

316.  Similar  effects  are  often  produced  in  nature.  Let  D  (see 
Fig.  148)  be  a  mass  of  rock  through  which  runs  a  long  crevice, 
A  B,  communicating  with  C,  a  large  cavity  below,  full  of  water, 
and  having  no  outlet.  When  a  shower  fills  the  crevice,  so  great 
a  pressure  may  be  generated  as  to  rend  the  rock  in  fragments.  It  is  in  this 
way  that  many  of  the  great  convulsions  of  nature  are  produced. 


What  should  be  the  strongest  part  of  dams,  sea-walls,  and  barrels,— and  why?  De- 
scribe the  experiment  which  proves  that  the  upward  pressure  of  liquids  increases 
with  their  depth.  314.  What  is  said  of  the  pressure  of  water  at  great  depths  ?  What 
experiment  is  often  made  with  strong  glass  bottles  ?  What  is  the  effect  of  this  pres- 
sure on  wood  sunk  to  a  great  depth  ?  315.  How  may  wonderful  effects  be  produced 
by  an  insignificant  body  of  liquid?  How,  for  example,  may  a  cask  be  burst? 
816.  What  similar  effect  is  produced  in  nature  ?  31T.  What  is  meant  by  the  Hydro- 


PRESSURE   OP  LIQUIDS. 


137 


317.  Hydrostatic  Fis- 148- 
Paradox. — Pressure 

being  proportioned 
to  depth  alone,  a  very 
small  quantity  of  li- 
quid may  balance  any 
quantity,  however 
great.  This  princi- 
ple is  called  the  Hy- 
drostatic Paradox. 
Improbable  as  it  appears  at  first,  its  truth  is  proved  in  va- 
rious ways. 

In  Fig.  149,  let  A  be  a  vessel  holding  50  gallons, 
and  B  a  tube  of  the  same  height,  communicating  with 
A,  and  having  a  capacity  of  one  gallon.  Water  poured 
in  either  rises  to  the  same  height  in  both.  When  both 
are  full,  the  pressure  of  the  one  gallon  in  the  tube  must 
be  as  great  as  that  of  the  50  gallons  in  the  vessel ;  oth- 
erwise, the  latter  would  force  its  way  into  the  tube  and 
cause  the  water  there  to  overflow. 

318.  Rule  for  finding  the  Pressure  on 
the  Bottom  of  a  Vessel. — To  find  the  pres- 
sure of  a  body  of  liquid  on  the  bottom  of 

the  vessel  containing  it,  multiply  its  height  into  the  area 
of  the  vessel's  bottom. 


Fig.  149. 


Fig.  150. 


According  to  this  rule,  different 
quantities  of  liquid  may  produce  equal 
pressure.  In  Fig.  150,  let  A,  B,  and  C 
be  three  vessels  having  equal  bases, 
and  containing  the  same  depth,  though 
different  quantities,  of  liquid  ;  then  the 
pressure  on  their  bottoms  will  be  equal. 

319.  Hydrostatic  Bellows.  —  Interesting  experiments 
may  be  performed  with  the  Hydrostatic  Bellows,  repre- 
sented in  Fig.  151. 


static  Paradox  ?  Prove  the  truth  of  the  paradox  with  the  apparatus  represented  in 
Fig.  149.  318.  What  is  the  rule  for  finding  the  pressure  of  a  body  of  liquid  on  the 
bottom  of  the  vessel  containing  it  ?  Explain  how  different  quantities  of  liquid  may 
produce  equal  pressure.  819.  Describe  the  Hydrostatic  Bellows,  and  the  experiment 


138 


HYDEOSTATICS. 


Fig.  151.  A  metallic  pipe,  about  four  feet  long,  is  screwed  into  a  water- 
tight apartment,  formed  of  two  circular  pieces  of  board  fastened 
together  with  a  broad  leather  band.  As  water  is  poured  into  the 
pipe,  the  top  of  the  bellows  rises,  and  with  such  force  as  to  lift 
heavy  weights  placed  upon  it.  When  both  pipe  and  bellows  are 
full,  the  latter  will  support  from  three  to  four  hundred  pounds. 
It  matters  not  how  small  the  bore  of  the  pipe  may  be ;  the  pres- 
sure depends  solely  on  its  height. 

320.  Hydrostatic  Press. — A  useful  application 
of  the  same  principle  is  made  in  Bramah's  Hydro- 
static (or  Hydraulic)  Press,  exhibited  in  Fig.  152. 

E  B  represents  a  forcing-pump  worked  by  the  lever  A.    This 
instrument,  which  is  fully  described  on  page  188,  consists  of  a 
piston  working  within  a  small  tube  to  which  it  is  tightly  fitted, 
and  which   descends,   as    shown  by  the  dotted  lines,  into    a 
cistern  in  the  bottom  of  the  frame  of  the  press.    F  G  is  a  tube 
connect- 
ing    EB 
with  the 
large  cyl- 
inder  C, 
to  which 
is    fitted 
a  smaller 
wrought- 

iroa  cylinder  D,  free  to  move  up 
and  down  within  it.  D  has  a  plat- 
en, H  H,  attached  to  it,  between 
which  and  the  top  of  the  frame, 
the  cotton,  hay,  cloth,  or  other 
substance  to  be  pressed,  is  placed. 
To  work  the  press,  raise  the 
long  arm  of  the  lever  A.  Water 
is  by  this  means  drawn  up  from 
the  cistern  into  the  tube  E  B ;  and, 
when  A  is  lowered  and  the  piston 
thus  made  to  descend,  being  pre- 
vented from  returning  to  the  cis- 
tern by  a  valve  which  closes,  it  is 
forced  through  the  tube  F  G  into 
the  lower  part  of  the  cylinder  C.  D  being  thus  driven  up  and  with  it  the 
platen,  whatever  is  confined  between  the  latter  and  the  top  of  the  frame  is 


HYDBOSTATIO    BELLOWS. 


HYDROSTATIC 


performed  with  it.  How  great  a  weight  will  it  support  ?  320.  Describe  the  Hydrostatic 
Press,  with  the  Figure.    How  is  it  worked  ?    How  great  pressure  may  be  obtained 


SPECIFIC  GRAVITY.  139 

subjected  to  pressure,  greater  or  less  according  to  the  quantity  of  water 
forced  into  C. 

With  the  Hydrostatic  Press  any  degree  of  pressure 
may  be  obtained  that  is  not  too  great  for  the  strength  of 
the  materials  employed.  The  machine  is  extensively  used, 
not  only  for  pressing,  but  also  for  extracting  stumps,  test- 
ing cables,  and  raising  vessels  out  of  water. 

Specific  Gravity. 

321.  If  we  weigh  a  cubic  inch  of  water,  and  then  the 
same  bulk,  or  volume,  of  silver,  and  of  cork,  we  find  the 
silver  heavier  than  the  water,  and  the  cork  lighter.     If  we 
proceed  to  compare  the  weights  of  various  other  substances, 
taking  a  cubic  inch  of  each,  we  shall  find  that  they  ah1  differ 
more  or  less.     To  express  the  comparative  weight  of  differ- 
ent  substances,  the  term  Specific  Gravity  is  used. 

322.  The  Specific  Gravity  of  a  substance  is  the  weight 
of  a  given  bulk  of  it  compared  with  the  weight  of  an  equal 
bulk  of  some  other  substance  taken  as  a  standard.     The 
standard  employed  is  distilled  water  at  the  temperature  of 
60  degrees. 

A  standard  of  this  kind  must  be  invariable.  Hence  the  temperature  of 
the  water  is  fixed ;  for  at  a  higher  degree  of  heat  it  would  become  rarer,— 
and  at  a  lower  degree,  denser.  Distilled  water  is  taken,  because  it  is  pure  } 
the  intermixture  of  vegetable  and  mineral  matter  in  spring  and  river  watef 
affects  their  density,  and  makes  them  unfit  for  a  standard. 

A  cubic  inch  of  silver  weighs  101/.,  times  as  much  as  a  cubic  inch  of  wa- 
ter; accordingly,  the  specific  gravity  of  water  being  1,  that  of  silver  is  lO1/^. 
A  cubic  inch  of  cork  weighs  24/ioo  as  much  as  the  same  bulk  of  water ;  the 
specific  gravity  of  cork,  therefore,  is  set  down  at  24/100  or  .24. 

323.  Fluids  that  do  not  mix,  when  brought  together, 
arrange  themselves  in  the  order  of  their  specific  gravities, 
the  heaviest  at  the  bottom.     Thus,  if  mercury,  water,  and 
oil  be  thrown  into  a  tumbler,  the  mercury  will  settle  at  the 


with  the  hydrostatic  press  ?  For  what  is  this  machine  used  ?  321.  If  we  weigh  equal 
bulks  of  different  substances,  what  do  we  find  ?  What  term  is  used  to  express  th« 
comparative  weight  of  different  substances  ?  322.  What  is  Specific  Gravity  ?  What 
Is  taken  as  a  standard?  Why  is  the  temperature  of  the  water  fixed?  Why  is  dis- 
tilled water  taken  ?  What  is  the  specific  gravity  of  silver,  and  why?  What  is  the 
Specific  gravity  of  cork,  and  why  ?  823.  How  do  fluids  that  do  not  mix,  when  brought 


140  HYDROSTATICS. 

bottom,  because  its  specific  gravity  is  greatest ;  next  will 
come  the  water ;  and  on  top,  the  oil,  which  is  the  lightest 
of  the  three. 

Cream  rises  on  milk,  because  its  specific  gravity  is  less  than  that  of  milk. 
For  the  same  reason,  the  oily  particles  of  soup  float  on  the  top. 

The  negroes  in  the  West  Indies  take  advantage  of  this  law  of  specific 
gravity.  When  they  want  to  steal  rum  out  of  a  cask,  they  introduce  through 
the  hole  in  its  top  the  neck  of  a  bottle  filled  with  water.  The  water  descends 
on  account  of  its  greater  weight,  and  rum  takes  its  place  in  the  bottle. 

324.  Gases,  like  liquids,  differ  in  their  specific  gravity.  Smoke  rises,  be- 
cause it  is  lighter  than  air.  Hydrogen  is  so  much  inferior  to  air  in  specific 
gravity,  that  it  not  only  rises  itself,  but  also  carries  up  a  loaded  balloon. 
Carbonic  acid  gas,  on  the  other  hand,  is  somewhat  heavier  than  air ;  it  is 
therefore  found  at  the  bottom  of  wells  and  mines,  where  its  poisonous  prop- 
erties sometimes  prove  fatal  to  those  who  descend. 

325.  If  a  solid  floats  on  a  liquid,  like  cork  on  water,  its 
specific  gravity  is  less  than  that  of  the  liquid ;  if  it  sinks, 
like  lead,  its  specific  gravity  is  greater.  If  solid  and  liquid 
have  the  same  specific  gravity,  the  solid  wifl  remain  sta- 
tionary at  any  depth  at  which  it  is  placed,  without  rising 
or  sinking. 

That  a  solid  may  float,  it  is  not  essential  that,  in  a  compact  mass,  it  weigh 
less  than  a  like  bulk  of  the  liquid.  A  solid  may  therefore  float  or  sink  in 
the  same  liquid,  according  to  the  form  it  is  made  to  assume.  A  cubic  inch 
of  iron  weighs  7J/4  times  as  much  as  a  like  bulk  of  water,  and  will  therefore 
sink  in  the  latter ;  but,  if  beaten  out  into  a  vessel  containing  more  than  7V* 
cubic  inches,  this  same  iron  will  float,  because  then  it  is  lighter  than  an  equal 
bulk  of  water.  It  is  on  this  principle  that  iron  ships  float. 

Fig.  153.  326.  A  floating  solid  displaces  its  own 

weight  of  liquid. 

To  prove  this,  fill  the  vessel  A  with  water  up  to  the 
opening  B.  Drop  in  a  ball  of  wood.  As  it  becomes 
partially  immersed,  it  raises  the  water  and  causes  it  to 
flow  through  B.  Catch  the  water  thus  displaced,  and 
it  will  be  found  to  weigh  exactly  the  same  as  the  ball. 

327.    A   body   immersed   in  water  is 

together,  arrange  themselves  ?  Give  an  example.  Why  does  cream  rise  on  milk  ? 
What  use  do  the  negroes  in  the  West  Indies  make  of  this  principle  ?  324.  What  is 
said  of  the  specific  gravity  of  gases  ?  Why  does  smoke  rise  ?  How  does  hydrogen 
compare  with  air  in  specific  gravity  ?  Carbonic  acid  ?  825.  When  will  a  solid  float 
on  a  liquid,  when  sink,  and  when  remain  stationary  without  rising  or  sinking  ?  How 
may  a  solid  which  in  a  compact  mass  is  heavier  than  water,  be  made  to  float? 


SPECIFIC   GEAVITY   OF  LIQUIDS.  141 

buoyed  up,  and  loses  as  much  weight  as  the  water  it  dis- 
places weighs. 

A  boy  can  bring  up  from  the  bottom  of  a  pond  a  heavy  stone  which  he 
could  not  lift  on  land.  In  raising  a  bucket  from  a  well,  we  find  it  become 
heavier  the  moment  it  leaves  the  water.  In  each  case,  the  weight  of  the  ob- 
ject, while  in  the  water,  is  diminished  by  its  upward  pressure. 

That    the  weight   thus    lost    equals  pig.  154. 

that  of  the  water  displaced,  is  shown 
with  the  apparatus  represented  in  Fig. 
154.  From  one  side  of  a  balance  sus- 
pend a  solid  cylinder  B,  and  on  the  same 
scale  place  a  hollow  cylinder  A,  which 
just  contains  the  other.  Balance  the 
whole  with  a  weight  C  in  the  opposite 
scale.  If,  now,  we  immerse  B,  still  sus- 
pended, in  a  vessel  of  water,  C  will  be 
found  to  outweigh  AB,  but  the  differ- 
ence is  exactly  made  up  by  filling  A  with 
water;  and  as  A  just  holds  B,  it  is  evi- 
dent that  it  holds  as  much  water  as  B  dis- 
places. 

328.  SPECIFIC  GRAVITY  OF  LIQUIDS. — The  specific  grav- 
ity of  a  body  is  simply  its  weight  compared  with  that  of  a 
like  bulk  of  water.  Hence  the  specific  gravity  of  a  liquid 
may  be  easily  obtained  in  the  following  way :  Fill  a  glass 
vessel,  whose  weight  is  known,  with  water  up  to  a  certain 
mark,  and  weigh  it ;  subtract  the  weight  of  the  vessel,  and 
you  have  the  weight  of  the  water  alone.  Then  fill  the  ves- 
sel to  the  same  height  with  the  liquid  in  question,  weigh  it 
again,  and  subtract  the  weight  of  the  vessel  as  before.  To 
find  the  specific  gravity  of  the  liquid,  divide  its  weight  by 
that  of  the  water. 

A  flask  that  will  hold  1,000  grains  of  water,  cafled  the  Thousand  Grain 
Bottle,  is  often  used  for  this  purpose.  A  glass  stopper,  with  a  narrow  open- 
ing running  lengthwise  through  it,  is  fitted  to  the  neck.  The  flask  being 
filled,  this  stopper  is  inserted ;  as  it  descends,  it  forces  out  the  excess  of 
liquid  through  its  opening,  and  thus  always  ensures  the  same  volume  of  liquid 

Give  an  example.  826.  How  much  liquid  does  a  floating  solid  displace  ?  Prove  this 
with  Fig.  158.  827.  How  much  weight  does  a  body  immersed  in  water  lose'?  Give 
•ome  familiar  examples  of  this  loss  of  weight.  Prove,  with  the  apparatus  represented 
in  Fig.  154,  that  the  weight  lost  equals  that  of  the  water  displaced.  828.  How  may 
the  specific  gravity  of  a  liquid  be  obtained?  What  is  the  Thousand  Grain  Bottle? 


142 


HYDKO  STATICS. 


inside.  A  flask  containing  1,000  grains  of  water  will  hold  13,568  grains  of 
mercury  and  792  grains  of  alcohol  ;  dividing  according  to  the  rule,  we  find 
the  specific  gravity  of  mercury  to  be  13.568,  and  that  of  alcohol  .792. 

329.  The  Hydrometer.  —  The  specific  gravity  of  liquids 
may  also  be  determined  by  the  Hydrometer.  This  instru- 
Fio.  155  ment  consists  of  a  hollow  ball,  C,  from  which 
rises  a  graduated  scale,  A  ;  while  to  its  lower 
side  is  attached  a  solid  ball,  B,  of  sufficient 
weight  to  keep  the  instrument  in  a  vertical  po- 
sition. 

To  find  the  specific  gravity  of  any  liquid,  place  the  hydrom- 
eter in  it.  The  rarer  the  liquid,  the  farther  it  descends  ;  and 
the  figure  on  the  scale  at  the  point  where  it  meets  the  surface, 
is  noted.  A  table  accompanies  the  instrument,  which  tells  the 
specific  gravity  of  a  liquid  when  the  height  to  which  it  rises 
on  the  scale  is  known. 

The  hydrometer  is  used  by  dealers  in  spirits,  oils,  and 
chemicals,  to  test  their  strength.  The  height  to  which  the 
pure  article  rises  on  the  scale  being  known,  any  different  re- 
sult when  a  liquid  is  tested,  indicates  adulteration. 

330.    SPECIFIC  GRAVITY  OP  SOLIDS.  —  The 

.  . 

simplest  way  of  finding  the  specific  gravity  of  a 
solid  would  be  to  take  a  certain  bulk  of  it  (say  a  cubic  inch 
or  cubic  foot),  ascertain  its  weight,  and  divide  it  by  the 
weight  of  a  like  bulk  of  water.  It  is  so  difficult,  however, 
to  obtain  any  given  bulk  exactly,  that  other  methods  have 
to  be  resorted  to. 

331.  If  the  solid  sinks  in  water,  weigh  it  first  in  air,  and 
then  in  water  by  means  of  a  balance  prepared  for  the  pur- 
pose. Divide  its  weight  in  air  by  the  weight  it  loses  in 
water,  and  the  quotient  will  be  its  specific  gravity. 

This  is  the  same  thing  as  dividing  the  weight  of  the  solid  by  that  of  an 
equal  bulk  of  water,  for  we  have  already  seen  that  a  solid  weighed  in  a  liquid 
loses  as  much  weight  as  the  liquid  it  displaces  weighs. 

How  many  grains  of  mercury  will  such  a  flask  hold  ?  Of  alcohol  ?  What,  then,  is 
the  specific  gravity  of  mercury  and  alcohol  ?  329.  What  instrument  is  used  for  ob- 
taining the  specific  gravity  of  liquids  ?  Describe  the  Hydrometer.  How  is  the  spe- 
cific gravity  obtained  with  this  instrument  ?  By  whom  is  the  hydrometer  chiefly 
used?  How  does  it  indicate  adulteration  ?  330.  What  would  be  the  simplest  mode 
of  finding  the  specific  gravity  of  a  soiid?  'What  difficulty  stands  in  the  way? 
831.  How  may  we  find  the  specific  gravity  of  a  solid  that  sinks  in  water  ?  Give  an 


THE  HYDBOM- 


SPECIFIC   GRAVITY   OF   SOLIDS.  143 

A  piece  of  platinum  weighs  22  grains  in  air,  and  21  in  water.  Dividing 
22,  the  weight  in  air,  by  1,  the  Joss  of  weight  in  water,  we  get  22  for  the  spe- 
cific gravity  of  platinum. 

332.  To  find  the  specific  gravity  of  a  solid  that  floats  on 
water,  attach  to  it  some  body  heavy  enough  to  sink  it. 
Weigh  the  two,  thus  attached,  in  air  and  in  water  ;  and  by 
subtraction  find  their  loss  of  weight  in  water.     In  the  same 
way,  find  how  much  weight  the  heavy  body  alone  loses  in 
water.     Subtract  this  from  the  loss  sustained  by  the  two, 
and  you  get  the  weight  of  a  volume  of  water  equal  to  the 
body  under  examination.    Divide  the  body's  weight  in  air 
by  this  remainder,  and  you  have  its  specific  gravity. 

Example.  Required  the  specific  gravity  of  a  piece  of  elm  wood  weighing 
2  ounces.  Attach  to  it  4  ounces  of  lead. 

The  combined  solids  weigh  in  air  2  +  4  =  6       ounces. 
In  water  we  find  them  to  weigh 3.15  ounces. 

Loss  of  the  combined  solids  in  water,    2.85  ounces. 

The  lead  alone  weighs  in  air 4      ounces. 

The  lead  alone  weighs  in  water 3.65  ounces. 

Loss  of  the  lead  in  water, 35  ounce. 

Weight  of  a  volume  of  water  equal  to  the  wood,  2.85  —  .35  =  2.50 
Specific  gravity  of  elm  wood,  2  -5-  2.50  =  .8 

333.  SPECIFIC  GEAVTTY  OF  GASES. — The  specific  gravity 
of  gases  is  found  by  a  process  similar  to  that  employed  for 
liquids.     Air  is  taken  for  the  standard.     A  glass  flask  fur- 
nished with  a  stop-cock  is  weighed  when  full  of  air,  and 
again  when  exhausted  by  means  of  an  air-pump  ;  the  differ- 
ence between  these  weights  is  the  weight  of  a  flask-full  of 
air.     The  flask  is  then  filled  with  the  gas  in  question,  and 
again  weighed ;  this  weight,  less  that  of  the  exhausted  flask, 
is  the  weight  of  a  flask-full  of  the  gas.    Divide  the  weight 
of  the  gas  by  that  of  the  air,  and  the  quotient  is  the  spe- 
cific gravity  required. 

334.  TABLES   OF  SPECIFIC  GRAVITIES. — The   following 


example.  332.  How  may  we  find  the  specific  gravity  of  a  solid  that  floats  on  water  ? 
Find  the  specific  gravity  of  a  piece  of  elm  wood  weighing  2  ounces.  833.  What  is 
taken  for  a  standard  in  estimating  the  specific  gravity  of  gases  ?  How  may  the  spe 


144 


HYDROSTATICS. 


tables  give  the  specific  gravity  of  some  of  the  most  impor- 
tant substances : — 

SPECIFIC  GRAVITY  OF  SOLIDS  AND  LIQUIDS.— Standard,  Distilled  Water,  1. 


Indium....  23.000 
Platinum...   22.069 

Gold 19.358 

Mercury....  13.568 

Lead 11.445 

Silver 10.474 

Copper,  cast    8.788 
Tin  . .  .     7.285 


Iron,  cast 7.207 

The  earth 5.210 

Diamond 3.536 

Parian  Marble  . .  2.838 
Anthracite  coal..  1.800 
Bituminous  coal.  1.250 
Lignum  vitae. . . .  1.333 
Oak..  .970 


Ice 930 

Living  men..     .891 

Cork 240 

Human  blood  1.045 

Milk 1.030 

Sea  water  . . .  1.026 

Olive  oil 915 

Alcohol  ..          .792 


SPECIFIC  GRAVITY  OF  GASES. — Standard,  Air,  1. 


Hydriodic  Acid 4.300 

Carbonic  Acid    1.524 

Oxygen   1.111 


Air    1.000 

Nitrogen 0.972 

Hydrogen    0.069 


335.  By  examining  the  above  tables,  it  will  be  found  that  solids  generally 
have  a  greater  specific  gravity  than  liquids,  and  liquids  than  gases.    Among 
solids,  the  metals  are  the  heaviest. 

The  heaviest  known  substance  is  the  metal  iridium,  which,  bulk  for  bulk, 
weighs  23  times  as  much  as  water.  The  lightest  substance  is  hydrogen  gas. 
'It  would  take  about  12,000  cubic  feet  of  hydrogen  to  weigh  as  much  as  one 
cubic  foot  of  water. 

Sea-water,  being  impregnated  with  salts,  is  somewhat  heavier  than  fresh 
water.  It  is  therefore  more  buoyant ;  and  this  every  swimmer  that  has  tried 
it  knows.  A  vessel  passing  from  fresh  water  to  the  sea,  draws  less  water  in 
the  latter,  that  is,  does  not  sink  to  so  great  a  depth. 

336.  Water  is  828  times  heavier  than  air ;  that  is,  it  would  take  828  cubic 
inches  of  air  to  weigh  as  much  as  1  cubic  inch  of  water.     Hence,  by  confin- 
ing air  in  tight  chambers  in  different  parts  of  life-boats,  they  are  made  so 
buoyant  that  they  can  not  sink  even  when  filled  with  water.    Life-preservers 
act  on  the  same  principle.    The  air  confined  in  them,  being  828  times  lighter 

cine  gravity  of  gases  be  found  ?  334.  [Questions  on  the  Tables. — "Which  is  the  densest 
of  the  metals  ?  Which  is  the  densest  of  liquids  ?  "Will  the  wood  called  lignum  vita 
float  in  water  ?  What  liquid  will  it  float  in  ?  Which  weighs  more,  a  cubic  foot  of 
water  or  the  same  bulk  of  ice  ?  In  which  would  a  boat  sink  deepest,  olive  oil,  alco- 
hol, or  sea- water  ?  Could  a  man  swim  in  alcohol  ?  Would  a  balloon  rise  most  easily 
in  hydrogen,  carbonic  acid,  or  air?  Would  a  balloon  filled  with  oxygen  rise  in  air?] 
335.  How  do  solids,  as  a  general  thing,  compare  with  liquids  in  specific  gravity  ?  How 
do  gases  compare  with  liquids  ?  Among  solids,  what  class  of  bodies  are  heaviest  ? 
What  is  the  heaviest  known  substance  ?  How  does  its  weight  compare  with  that  of 
water?  What  is  the  lightest  substance?  How  many  cubic  feet  of  hydrogen  would 
it  take  to  weigh  as  much  as  one  cubic  foot  of  water  ?  How  does  sea-water  compare 
with  fresh  water  in  specific  gravity?  In  which  is  it  easier  to  swim  ?  In  which  does 
a  vessel  draw  less  water  ?  836.  How  does  air  compare  with  water  in  specific  gravity  ? 


SPECIFIC   GRAVITY.  145 

than  the  same  bulk  of  water,  helps  to  keep  up  the  bodies  to  which  they  may 
be  attached.  Many  species  of  fish  are  provided  with  bladders,  which  they 
can  fill  with  air  or  exhaust  at  pleasure  ;  they  are  thus  able  to  increase  or  di- 
minish their  specific  gravity  instantaneously,  and  to  rise  or  sink  accordingly. 
337.  The  specific  gravity  of  living  men  is  set  down  at  .891,  or  less  than  9/10 
of  that  of  water.  The  human  body,  therefore,  will  float ;  and,  if  the  head  is 
thrown  back  so  as  to  bring  the  mouth  uppermost,  there  is  no  danger  of  drown- 
ing, even  in  the  case  of  those  who  can  not  swim.  If  the  air  is  expelled  from 
the  lungs,  and  water  takes  its  place,  the  specific  gravity  is  increased  ;  conse-i 
quently  the  bodies  of  drowned  persons  sink.  After  remaining  under  water 
for  a  time,  they  again  float ;  this  is  owing  to  the  generation  of  light  gases 
within  them,  by  which  their  specific  gravity  is  lessened. 

338.  If  we  know  the  specific  gravity  of  a  body,  we  can 
easily  find  how  much  any  given  bulk  of  it  weighs.    A  cubic 
foot  of  water  is  found  to  weigh  1,000  ounces,  or  62£  pounds 
avoirdupois ;  the  weight  of  a  cubic  foot  of  any  given  sub- 
stance will,  therefore,  be  equal  to  62^  pounds  multiplied  by 
its  specific  gravity. 

Example.  Required  the  weight  of  a  cubic  foot  of  gold.  The  table  makes 
the  specific  gravity  of  gold  19.358.  Multiplying  this  into  62.5,  we  get 
1209.875  pounds  for  the  weight  required. 

339.  Two  solids  of  equal  bulk  will  displace  equal  quan- 
tities of  a  liquid  in  which  they  are  immersed ;  but  two  sol- 
ids of  equal  weight  will  not  do  so,  unless  their  specific  grav- 
ity is  the  same.     This  principle  has  been  applied  in  testing 
the  purity  of  the  precious  metals. 

If,  for  instance,  we  wish  to  find  whether  a  piece  of  silver  is  pure,  we  put 
it  in  a  vessel  even  full  of  water,  and  catch  what  overflows  :  we  do  the  same 
with  an  equal  weight  of  what  is  known  to  be  pure  silver.  If  equal  quantities 
of  water  are  displaced,  the  article  tested  is  pure,  for  it  has  the  same  specific 
gravity  as  pure  silver ;  but  if  not,  it  is  adulterated. 

340.  The  fact  above  stated  was  discovered  and  .first  applied  by  Archime- 
des. Hiero,  king  of  Syracuse,  having  purchased  a  golden  crown  and  sus- 
pecting the  purity  of  the  metal,  asked  the  philosopher  to  test  it,  without  in- 
jury to  its  costly  workmanship.  In  vain  Archimedes  tried  to  solve  the  prob- 

On  what  principle  are  life-boats  and  life-preservers  constructed  ?  How  are  fish  ena- 
bled to  rise  or  sink  at  pleasure  ?  337.  How  does  the  body  of  a  living  man  compare 
with  water  in  specific  gravity  ?  What  follows,  as  regards  danger  of  drowning  ?  Why 
do  the  bodies  of  drowned  persons  at.  first  sink,  and  afterwards  rise  ?  338.  If  we  know 
the  specific  gravity  of  a  body,  how  may  we  find  the  weight  of  any  given  bulk  of  it  ? 
Give  an  example.  839.  When  will  two  solids  immersed  in  a  liquid  displace  equal 
quantities?  To  what  has  this  principle  been  applied?  How,  for  example,  may  wo 
find  whether  a  piece  of  silver  is  pure  ?  840.  By  whom  was  this  principle  discovered  ? 

7 


146  HYDROSTATICS. 

lem ;  till  one  day,  when  bathing,  he  observed,  that,  as  more  and  more  of  his 
body  became  submerged,  the  water  rose  proportionally  higher  and  higher  in 
the  vessel.  It  at  once  occurred  to  him  that  any  body  of  equal  weight  and 
exactly  the  same  density,  but  no  other,  would  cause  an  equal  rise  of  the  liquid ; 
and  here  was  a  clue  to  the  solution  of  the  problem  that  had  troubled  him. 
Naked  as  he  was,  he  rushed  home  from  the  bath,  shouting  "  Heureka  ! "  / 
have  found  it  !  He  immediately  procured  a  quantity  of  pure  gold  equal  in 
weight  to  the  crown,  and  a  like  weight  of  pure  silver.  Then  successively 
plunging  the  gold,  the  silver,  and  the  crown,  in  a  vessel  brim-full  of  water, 
he  caught  and  weighed  the  liquid  displaced  in  each  case.  Finding  that  the 
crown  displaced  more  than  the  gold  and  less  than  the  silver,  he  inferred  tbat 
it  was  neither  pure  gold  nor  pure  silver,  but  a  mixture  of  the  two.  Archi- 
medes afterwards  investigated  the  subject  further,  and  discovered  the  leading 
principles  connected  with  specific  gravity. 

Capillary  Attraction. 

341.  If  one  end  of  a  fine  glass  tube  be  placed  in  a  ves- 
sel of  water,  the  other  end  being  left  open,  the  water  will 
rise  in  ttie  tube  above  its  level.     The  force  that  causes  the 
water  to  rise  is  known  as  Capillary  Attraction.     It  is  so 
called  from  the  Latin  word  capillus,  a  hair,  because  it  is 
most  strikingly  exhibited  in  tubes  as  fine  as  a  hair. 

A  liquid  will  not  rise  by  capillary  attraction  in  tubes 
that  exceed  one-fifteenth  of  an  inch  in  diameter. 

342.  CAUSE. — The  rise  of  liquids  in  capillary  tubes  is 
owing,  it  is  thought,  to  the  attraction  of  the  inner  surface 
of  the  solid.     In  proof  of  this,  we  find  that  the  surface  of 
the  liquid  in  the  tube  is  concave,  being  raised  where  it 
comes  in  contact  with  the  sides  of  the  tube. 

Fio>  15g  The  same  thing  is  seen  when  a 

glass  plate,  C,  is  placed  perpendicu- 
larly  in  water,  A  B :  the  surface,  in- 
stead of  maintaining  the  same  level 

- g      throughout,  rises  near  the  glass  on 

both  sides,  as  represented  by  the 
dotted  lines. 

The  above  experiment  seems  to 
show  that  the  attraction  of  glass  for  water  is  sufficiently  great  to  overcome 

Relate  the  circumstances.  341.  What  is  Capillary  Attraction  ?  Why  is  it  so  called  ? 
"What  is  the  limit  of  size  for  capillary  tubes?  342.  To  what  is  the  rise  of  a  liquid  in 
capillary  tubes  owing?  What  proof  is  there  of  thi??  When  a  glass  plate  is  placed 
perpendicularly  in  water,  what  may  be  observe*?  ?  "What  does  this  experiment  show  ? 


CAPILLARY   ATTRACTION.  147 

the  gravity  of  the  latter.  It  is,  also,  greater  than  the  cohesion  subsisting 
between  the  particles  of  water;  for,  if  the  glass  be  removed,  some  of  the 
liquid  will  be  found  adhering  to  its  surface,— that  is,  it  will  be  wet. 

343.  This  attraction,  however,  does  not  exist  between 
all  solids  and  liquids  ;  on  the  contrary,  we  sometimes  find 
as  decided  a  repulsion. 

Let  the  glass  plate,  for  instance,  in  the  last  experiment,  be  greased,  and 
the  water,  now  acted  on  by  a  repelling  force,  instead  of  being  elevated  near 
the  sides,  will  be  depressed,  as  Fig.l5T. 

shown  by  the  dotted  lines  in  Fig. 
157.  A  similar  appearance  is  pre- 
sented when  a  glass  plate  is  plunged 

into  a  dish  of  mercury.    When  this     J) - 

repulsion  exists,  the  liquid  does  not 
wet  the  solid ;  when  the  glass  plate 
is  drawn  out  of  the  mercury,  not  a 
particle  of  the  liquid  adheres  to  it. 

The  repulsion  just  mentioned  may  be  so  great  as  to  prevent  a  solid  from 
sinking  in  a  liquid  lighter  than  itself.  A  fine  needle  smeared  with  grease,  if 
carefully  laid  in  a  horizontal  position  on  the  surface  of  still  water,  will  re- 
main floating  there.  It  is  thus  that  insects  are  able  to  walk  on  water;  the 
repulsion  between  their  feet  and  the  liquid  prevents  them  from  sinking  or 
even  becoming  wet. 

344.  FAMILIAR  EXAMPLES. — Examples  of  capillary  at- 
traction meet  us  on  all  sides. 

If  one  end  of  a  towel  be  left  in  a  basin  of  water,  the 
part  outside  soon  becomes  wet,  the  liquid  being  drawn  up 
through  its  minute  fibres.  The  same  thing  happens  if  a 
piece  of  sponge,  of  bread,  or  of  sugar,  remains  in  contact 
with  a  liquid,  the  pores  of  the  substance  acting  like  capil- 
lary tubes.  Blotting  paper  drinks  up  ink  on  the  same 
principle. 

The  common  lamp  affords  a  good  illustration  of  capillary  attraction.  The 
oil  or  burning-fluid  is  drawn  up  through  the  fibres  of  the  wick  fast  enough 
to  support  the  flame.  There  is  a  limit,  however,  beyond  which  capillary  at- 
traction does  not  act ;  and,  therefore,  if  the  oil  gets  low,  the  lamp  grows 
dim  and  finally  goes  out.  To  allow  a  free  passage  to  the  oil,  the  little  tubes 

843.  What  sometimes  takes  the  place  of  this  attraction  between  solid  and  liquid  sur- 
faces? Give  an  example.  When  a  glass  plate  is  plunged  into  a  dish  of  mercury,  what 
phenomenon  is  presented  ?  What  is  sometimes  the  consequence  of  this  repulsion  ? 
Give  an  example.  How  is  it  that  insects  walk  on  water  ?  844.  How  may  capillary 
attraction  be  illustrated  with  a  towel  and  a  piece  of  bread  or  sugar  ?  How  is  the  flam» 


148 


HYDEOSTATICS. 


must  be  kept  clear  ;  and,  as  impurities  gather  in  them  from  the  ascending 
liquid,  the  wick  must  be  changed  from  time  to  time. 

Capillary  attraction  is  strikingly  exhibited  in  wood.  Water  is  drawn  up 
into  its  pores,  distending  them,  and  causing  a  perceptible  increase  of  size. 
This  expansion  is  turned  to  practical  account  in  the  south  of  France.  A 
large  cylinder  of  free-stone,  several  feet  in  length,  has  circular  grooves  made 
at  intervals  in  its  surface.  Into  these  grooves  are  driven  wedges  of  dry 
wood,  which  are  then  kept  wet  with  water.  As  the  wood  absorbs  the  liquid, 
it  gradually  expands,  till  it  rends  the  solid  cylinder  into  rough  mill-stones, 
which  require  but  little  labor  to  fit  them  for  market. 

It  is  capillary  attraction  that  renders  the  banks  of  streams  so  productive  ; 
the  water  drawn  in  through  the  pores  of  the  earth,  fertilizes  the  adjacent 
parts.  On  the  same  principle,  a  potted  plant  may  be  supplied  with  the  ne- 
cessary moisture  by  filling  the  saucer  in  which  it  stands  with  water.  Houses 
are  rendered  damp  by  the  absorption  of  external  moisture,  the  pores  of  the 
brick  or  stone,  of  which  the  walls  are  built,  acting  as  capillary  tubes. 

345.  LAWS  OF  CAPILLARY  ATTRACTION.  —  Different  li- 
quids rise  to  different  heights  in  tubes  of  the  same  size. 
Ether,  for  example,  rises  about  one-half,  and  sulphuric  acid 
only  one-third,  as  high  as  water. 

The  same  liquid  always  rises  to  the  same  height  in  a 
tube  of  given  size  ;  and  this  height  is  proportioned  to  the 
fineness  of  the  bore.  In  a  tube  T^  of  an  inch  in  diameter, 
water  rises  5T3¥  inches. 


Fig.  15a 


Fig.  159. 


346.  Fig.  158  represents  six  tubes  of 
different  bore,  communicating  at  the  bot- 
tom with  a  vessel  containing  colored  wa- 
ter.   The  water  rises  according  to  the  fineness  of  the  bore,  standing  highest 
in  the  smallest  tube. 


of  a  lamp  supplied  with  fuel  ?  How  is  capillary  attraction  exhibited  in  wood  ?  W  hat 
use  is  made  of  this  principle  in  France  ?  What  is  the  effect  of  capillary  attraction  on 
the  banks  of  streams  ?  How  may  a  potted  plant  be  supplied  with  moisture  ?  How 
lire  houses  made  damp  ?  845.  What  is  the  law  of  capillary  attraction,  as  regards  dif- 


CAPILLARY   ATTRACTION.  149 

347.  The  same  principle  is  illustrated  with  two  glass  plates  (see  Fig.  159), 
joined  at  one  end  and  slightly  diverging  so  as  to  form  an  angle  of  about 
two  degrees.  Let  the  plates  rest  in  colored  water  to  the  depth  of  an  inch,  and 
the  liquid  will  rise  between  them,  reaching  the  greatest  height  where  the 
surfaces  are  nearest  together,  and  thus  forming  the  curve  called  the  hy-per'- 
bo-la. 

348.  INTERESTING  FACTS. — If  a  capillary  tube  capable 
of  raising  water  four  inches  be  broken  off  at  three,  there 
will  be  no  overflow,  as  might  be  expected.     The  water  will 
rise  three  inches  to  the  top  of  the  tube,  and  there  stop. 
But  it  will  be  supplied  as  fast  as  evaporation  takes  place. 
Hence,  to  prevent  waste  in  a  spirit  lamp,  an  extinguisher 
is  put  over  the  wick  when  it  is  not  burning. 

It  is  a  remarkable  fact  that  no  evaporation  takes  place  unless  the  liquid 
reaches  the  top  of  the  capillary  tube.  Tubes  containing  as  much  water  as  they 
could  hold  under  the  influence  of  capillary  attraction,  have  been  hung  in  the 
sun  for  months,  without  losing  any  part  of  their  contents  by  evaporation. 

349.  FLOATING  BODIES. — Motion  is  produced  in  bodies 
floating  near  each  other,  by  a  force  resembling  capillary 
attraction.     This  may  be  shown  with  two  balls,  as  repre- 
sented in  Figs.  160,  161,  162. 

A  and  B  are  cork  balls,  capable  of  being  wet  Fig.  160. 

with  water.  When  they  are  brought  close  to- 
gether, the  attraction  of  their  surfaces  raises  the 
water  around  them;  the  column  that  separates 
them  becomes  thinner  and  thinner,  till  at  last 
they  touch. 

C  and  D  are  similar  balls,  greased  so  that  Fig.  16L 

they  can  not  be  wet.  In  this  case,  the  surface 
of  the  surrounding  water  is  repelled,  forming 
little  hollows  in  which  they  rest.  Since  there 
is  not  enough  liquid  between  them  to  balance 
the  pressure  from  without,  the  balls  again  ap- 
proach each  other. 

ferent  liquids  ?  Give  an  example.  Wh^t  is  the  law  for  the  same  liquid  in  a  tube  of 
given  size  ?  How  high  does  water  rise  in  a  tube  Vioo  of  an  inch  in  diameter  ?  346.  What 
does  Fig.  158  represent?  347.  Describe  the  experiment  with  two  glass  plates. 
848.  What  fact  is  stated  respecting  a  capillary  tube  broken  off  at  the  top  ?  Why  is  it 
necessary  to  put  an  extinguisher  on  a  spirit-lamp  ?  What  fact  is  stated  respecting 
evaporation  from  capillary  tubes  ?  349.  How  are  floating  bodies  affected  by  a  force 
resembling  capillary  attraction  ?  What,  for  example,  is  the  effect  on  cork  balls  capa- 
ble of  being  wet?  On  balls  greased  so  that  they  can  not  be  wet?  On  balls,  one  of 


150  HYDROSTATICS. 

Fig.  162.  E  and  F  are  a  pair  of  similar  balls,  one  of 

which,  E,  can  be  wet,  while  the  other,  F,  can 
not.  The  water,  attracted  by  E,  rises  around  it, 
whereas  around  F  it  is  depressed.  If  these  balla 
are  placed  near  together,  F,  being  repelled  from 
the  wall  of  water  around  E,  will  recede  from  it. 

350.  ENDOSMOSE  AND  EXOSMOSE. — Two  peculiar  results 
of  capillary  attraction,  known  as  Endosmose  and  Exosmose, 
remain  to  be  mentioned.  * 

Endosmose  is  the  inward  motion  of  a  fluid,  through  a 
membranous  or  porous  substance,  into  a  vessel  containing 
a  different  fluid.  Exosmose  is  the  outward  motion  of  the 
contained  fluid  through  the  same  substance. 

Fill  a  vessel  with  alcohol,  tie  over  the  top  a  bladder  that  has  been  soaked 
in  water,  and  immerse  the  whole  in  water.  In  a  few  hours  it  will  be  found 
that  water  has  passed  into  the  vessel  through  the  bladder,  and  that  alcohol 
has  passed  out  into  the  water.  The  former  movement  is  called  Endosmose ; 
the  latter,  Exosmose.  The  inward  current  is  stronger  than  the  outward  one. 
Water  passes  in  faster  than  alcohol  escapes  ;  and  consequently  the  bladder 
soon  becomes  puffed  out.  All  membranous  and  porous  substances,  such  as 
india  rubber,  plaster  of  paris,  wood,  &c.,  permit  the  passage  of  these  cur- 
rents, which  are  owing  to  capillary  attraction. 

351.  Endosmose  and  exosmose  are  exhibited  hi  the  case 
of  gases,  as  well  as  liquids. 

If  a  phial  full  of  air,  with  a  piece  of  thin  bladder  tied  over  its  mouth,  be 
placed  in  a  jar  of  carbonic  acid  gas,  the  latter  will  force  its  way  into  the 
phial  while  air  will  pass  out.  Here,  again,  the  inward  current  is  the  stronger ; 
the  bladder  is  puffed  out,  and  finally  bursts. 

The  facility  with  which  gases  thus  pass  in  and  out  through  porous  sub- 
stances is  proportioned  to  their  rarity.  Hydrogen,  the  rarest  of  known 
bodies,  exhibits  these  movements  in  their  greatest  perfection.  This  is  the 
reason  why  the  rose  balloons,  recently  so  popular  as  toys,  lose  their  buoy- 
ancy in  a  few  days.  They  are  made  of  thin  india  rubber,  and  filled  with  hy- 
drogen. When  allowed  to  remain  in  the  air,  endosmose  and  exosmose  take 
place.  Hydrogen  passes  out  through  the  pores  of  the  rubber,  and  air  takes 
its  place.  The  balloon  gradually  becomes  less  buoyant,  ceases  to  rise,  and  at 
last,  as  it  loses  more  of  its  hydrogen,  is  carried  to  the  ground  by  the  weight 
of  the  india  rubber. 

which  can  be  wet  and  the  other  not?  850.  What  is  Endosmose  ?  "What  is  Exosmose  ? 
Show  how  endosmose  and  exosmose  operate.  Through  what  sort  of  substances  do 
they  take  place?  851.  What,  besides  liquids,  are  affected  by  these  movements? 
Give  an  example.  What  gases  most  readily  pass  in  and  out  through  porous  sub- 
stances ?  What  gas  exhibits  endosmose  and  exosmose  most  distinctly  ?  What  is  the 


EXAMPLES   FOE  PRACTICE.  151 

352.  The  skin  being  porous,  a  liquid  with  which  it  re- 
mains in  contact  will  find  its  way  through  by  endosmose 
and  be  absorbed  by  the  body.     If  a  drop  of  the  powerful 
poison  called  prussic  acid  be  placed  on  the   arm,  a  suffi- 
cient quantity  to  cause  death  will  thus  be  taken  into  the 
system. 

353.  Endosmose  and  exosmose  enter  largely  into  the 
operations  of  nature.    They  cause  the  ascent  and  descent 
of  sap  in  trees  and  vines.     The  inside  of  living  plants  con- 
sists of  minute  cells,  containing  fluids  of  different  densities. 
These  fluids  are  constantly  passing  in  and  out  through  the 
porous  walls  which  separate  them,  under  the  influence  of 
exosmose  and  endosmose,  modified  by  the  vital  action  at 
the  same  time  going  on. 

EXAMPLES  FOB  PRACTICE. 

1.  (See  §  328.)  A  phial  weighing  4  ounces  when  empty,  weighs  6  ounces  when 

filled  with  water,  and  7  when  filled  with  nitric  acid.    Required,  the  spe- 
cific gravity  of  the  acid. — Ans.  1.5. 

2.  A  vessel  filled  with  ether  weighs  13.575  ounces  ;  filled  with  water,  15 

ounces  ;  when  empty,  10  ounces.     What  is  the  specific  gravity  of  ether  ? 

3.  An  empty  jar  weighs  7.5  pounds ;  filled  with  sulphuric  acid,  it  weighs 

12.1125  pounds ;  and  filled  with  water,  10  pounds.    Find  the  specific 
gravity  of  sulphuric  acid. 

4.  A  Thousand  Grain  Bottle  is  found  to  hold  870  grains  of  oil  of  turpentine, 

and  1,036  grains  of  oil  of  cloves.    What  is  the  specific  gravity  of  these 
oils? 
In  which  would  a  cork  ball  sink  the  deeper  ? 

5.  (See  §  331.)  A  piece  of  crown-glass  weighs  5  ounces  in  the  air,  and  3  in 

water.    What  is  its  specific  gravity  ?— Ans.  2.5. 

6.  A  beef-bone  weighs  2.6  ounces  in  water,  and  6.6  ounces  in  air.    What  is 

its  specific  gravity? 

7.  What  is  the  specific  gravity  of  a  piece  of  ivory,  which  weighs  16  ounces 

in  air,  and  loses  83/4  ounces  when  weighed  in  water? 

8.  (  To  solve  the  next  two  sums,  see  §  332  and  Example.    In  each  case,  we  may 

suppose  a  pound  (16  ounces}  of  lead,  weighing  14.6  ounces  in  water,  to  be 
used  for  sinking  the  solid.} 
A  piece  of  wax  weighs  8  ounces ;  when  it  is  fastened  to  a  pound  of 

effect  of  these  movements  on  rose  balloons?  852.  "What  is  their  effect,  when  a  liquid 
is  placed  on  the  skin  ?  Give  an  example.  853.  What  is  the  effect  of  endosmose  and 
exosmose  in  trees  and  vines  ? 


152  HYDRAULICS. 

lead,  the  whole  weighs  in  water  13.712  ounces.    What  is  the  specific 
gravity  of  the  wax?— Ans.  .9. 

9.  Fastening  a  piece  of  ash  to  a  pound  of  lead,  I  find  their  weight  in  water 

to  be  12.76  ounces.    The  ash  alone  weighs  10  ounces  in  the  air.    What 
is  its  specific  gravity  ? 

10.  (See  §  333.)  A  glass  flask,  with  the  air  exhausted,  weighs  4  ounces ;  filled 
with  air,  it  weighs  4.25  ounces ;  and  filled  with  cy-an'-o-gen,  4.45125  oz. 
What  is  the  specific  gravity  of  cyanogen  t—Ans.  1.805. 

11.  A  flask  full  of  chlorine  weighs  11.222  ounces.    Filled  with  air,  it  weighs 
10.5-oz.,  and  when  the  air  is  drawn  out,  10  oz.    Required,  the  specific 
gravity  of  chlorine. 

12.  According  to  the  answers  of  the  last  two  sums,  in  which  would  a  balloon 
rise  most  easily,  air,  cyanogen,  or  chlorine  ? 

13.  (See  §  336.)  How  many  cubic  feet  of  air  would  it  take  to  weigh  as  much 
as  4  cubic  feet  of  water  ? 

14.  (See  §338,  and  Table.)  How  much  would  a  cubic  foot  of  gold  weigh? 
How  much,  the  same  bulk  of  silver  ? 

15.  What  would  be  the  weight  of  4  cubic  feet  of  Parian  marble? 

16.  What  is  the  weight  of  a  block  of  anthracite  coal,  6  feet  long,  4  feet  wide, 
and  3  feet  Ligh  ?    ( To  find  the  number  of  cubic  feet  in  the  block,  multiply 
the  length,  breadth,  and  thickness  together.) 

17.  Suppose  a  room  10  feet  high,  long,  and  wide,  to  be  filled  with  gold,  what 
would  the  gold  weigh  ? 


CHAPTER  XI. 
MECHANICS   (CONTINUED). 

HYDRAULICS. 

354.  HYDRAULICS  treats  of  liquids  in  motion,  whether 
issuing  from  orifices  or  running  in  pipes  and  the  beds  of 
streams.    It  shows  how  water  is  applied  as  a  moving  power, 
and  describes  the  machines  used  for  raising  liquids. 

355.  FLOW  OF  LIQUIDS  THROUGH  ORIFICES. — If  an  orifice 
be  made  in  the  side  or  bottom  of  a  vessel  containing  a  liquid, 
the  latter  will  escape  through  it.     The  particles  of  liquid 
near  the  orifice  are   forced  out  by  the  pressure  of  those 
above. 


FLOW   OF   LIQUIDS  THROUGH   ORIFICES.  153 

356.  Velocity. — The  velocity  of  a  stream  flowing  through 
an  orifice  depends  on  the  distance  of  the  latter  below  the 
surface  of  the  liquid,  being  equal  to  the  velocity  which  a 
body  would  acquire  in  falling  that  distance. 

If,  for  instance,  in  a  reservoir  full  of  water,  three  orifices  be  made  at 
depths  of  ISVia,  64y3,  and  1443/4  feet,  the  liquid  (leaving  friction,  Ac.,  out  of 
account)  will  issue  from  them  with  velocities  of  321/8,  64>/3,  and  96Va  feet  per 
second,  because  such,  as  we  have  found,  would  be  the  velocity  of  a  body  fall- 
ing through  the  different  distances  first  named. 

The  distances  above  mentioned  are  to  each  other  as  1,  4,  9  ;  the  velocities 
are  to  each  other  as  the  square  roots  of  these  numbers,  1,  2,  3.  Consequent- 
ly, the  velocities  of  streams  issuing  from  different  orifices  in  the  same  vessel 
are  to  each  other  as  the  square  roots  of  their  respective  distances  below  the  sur- 
face of  the  liquid.  Friction,  however,  and  other  causes,  produce  more  or 
less  deviation  from  this  rule. 

357.  As  long  as  the  liquid  is  kept  at  the  same  height  in 
the  vessel,  it  issues  from  a  given  orifice  with  the  same  ve- 
locity ;  but,  if  the  vessel  is  not  replenished,  as  the  liquid 
gets  lower,  the  pressure  diminishes,  and  the  velocity  of  the 
stream  diminishes  with  it.    It  takes  twice  as  long  to  empty 
an  unreplenished  vessel  through  a  given  orifice,  as  it  would 
for  the  same  quantity  of  water  to  escape  if  the  liquid  were 
kept  at  its  original  level. 

358.  The  Clepsydra. — Among  the  ancients,  time  was 
measured  by  the  flow  of  water  through  an  orifice,  in  an  in- 
strument called  the  Clepsydra,  or  Water-clock.    It  consist- 
ed of  a  transparent  vessel  with  a  hole  in  the  bottom  that 
would  empty  it  in  a  certain  time.    A  scale  on  the  side  of 
the  vessel  indicated,  by  figures  at  different  levels,  the  num- 
ber of  hours  which  it  took  the  liquid  to  reach  them  suc- 
cessively in  its  descent.    As  the  discharge  was  most  rapid 
when  the  vessel  was  full,  the  divisions  were  of  course  longest 
at  the  top  of  the  scale. 

The  clepsydra  was  necessarily  inaccurate,  inasmuch  as  the  flow  of  the 

354.  Of  what  does  Hydraulics  treat?  855.  What  causes  a  liquid  to  flow  through 
an  orifice  in  the  vessel  containing  it  ?  356.  On  what  does  the  velocity  with  which  a 
Btream  issues  from  an  orifice  depend  ?  Give  an  example.  What  is  the  law  for  the 
Velocities  of  streams  issuing  from  different  orifices  in  the  same  vessel  ?  357.  What 
difference  does  it  make,  as  regards  the  velocity  of  a  stream  through  an  orifice,  whether 
the  vessel  is  kept  replenished  or  not  ?  358.  What  did  the  ancients  use  for  measuring 

7* 


154  HYDRAULICS. 

water  varied  in  rapidity  according  to  its  temperature  and  the  density  of  the 
atmosphere.  Yet  it  answered  for  general  purposes  ;  indeed,  it  was  the  only 
instrument  used  for  measuring  small  intervals  of  time  in  astronomical  ob- 
servations. 

359.  « Course  of  Streams  flowing  from  Orifices. — A 
liquid  issuing  from  an  orifice  descends  in  the  same  line  as 
a  projectile  (see  §  127).  The  curve  described  is  called  a 
parabola.  In  a  given  vessel,  a  stream  will  spout  to  the 
greatest  horizontal  distance,  from  an  orifice  midway  be- 
tween the  surface  and  the  bottom  of  the  liquid.  Streams 
flowing  through  orifices  equally  removed  from  this  central 
one,  will  spout  to  the  same  distance. 

Fig.  168.  ID  ^l£- 163,  if  the  orifice  B  be  midway  between 

the  surface  and  the  bottom  of  the  liquid,  the  stream 
passing  through  it  will  spout  to  the  greatest  dis- 
tance ;  and  if  A  and  C  be  equi-distant  from  B,  the 
streams  passing  through  them  will  reach  the  same 
point, 

360.  Volume  discharged. — To  find 
the  volume  of  liquid  discharged  in  a 
given  time  from  an  orifice  in  a  vessel 
that  is  kept  replenished,  multiply  the  area  of  the  orifice  by 
the  velocity  of  the  stream  per  second,  and  this  product  by 
the  number  of  seconds. 

No  allowance  is  here  made  for  friction;  in  practice, 
therefore,  the  discharge  is  less  than  would  appear  from 
this  rule. 

Example.  How  much  water  will  be  discharged  from  an  orifice  of  2  square 
inches  in  5  seconds,  the  velocity  of  the  stream  being  10  inches  in  a  second, 
and  the  vessel  being  kept  replenished  ? — Ans,  2  X  10  X  5  =  100  cubic  inches. 

361.  The  quantity  discharged  through  a  given  orifice 
in  a  given  time  differs  in  the  case  of  different  liquids.  Al- 
cohol, for  instance,  flows  more  slowly  than  water,  and  mer- 

time  ?  Describe  the  clepsydra.  What  rendered  the  clepsydra  inaccurate  ?  859.  What 
curve  does  a  stream  issuing  from  an  orifice  describe  ?  At  what  part  of  a  vessel  will  a 
stream  from  an  orifice  spout  to  the  greatest  distance  ?  What  is  said  of  streams  equal- 
ly removed  from  the  central  one?  Exemplify  these  principles  with  Fig.  163. 

860.  What  is  the  rule  for  finding  the  volume  of  liquid  discharged  from  an  orifice  in  a 
given  time  ?    What  causes  deviations  from  this  rule  in  practice  ?    Give  an  example, 

861.  What  is  said  of  the  quantity  discharged  in  the  case  of  different  liquids  ?    Give  an 


FLOW   OF  LIQUIDS  THEOUGH   OEIFICES. 


155 


cury  more  rapidly ;  the  discharge  of  alcohol  will  therefore 
be  less,  and  that  of  mercury  greater,  than  the  discharge 
of  water. 

362.  A  circular  orifice  of  given  area  discharges  more 
liquid  in  a  given  time  than  one  of  any  other  shape.  This  is 
because  a  circle  is  the  smallest  line  that  can  enclose  a  given 
space ;  in  passing  through  a  circular  orifice,  therefore,  the 
liquid  comes  in  contact  with  a  less  extent  of  solid  surface, 
and  is  less  retarded  by  friction. 

363.  The  volume  discharged  through  an  orifice  in  a  given  time  may  be 
increased  by  heating  the  liquid.    Heat  lessens  its  cohesion,  and  enables  it  to 
flow  more  rapidly. 

364.  The  discharge  may  also  be  increased  by  fitting  a  short  tube,  or  Ad- 
jutage, to  the  orifice.    The  minute  currents  of  the  particles  are  thus  prevent- 
ed from  obstructing  each  other  in  the  act  of  passing  out.     The  best  shape  for 


such  a  tube  is  that  of  a  bell  with  the  large 
end  out,  as  shown  at  A  in  Fig.  164.  When 
such  a  tube  is  used,  the  discharge  in  a  given 
time  is  increased  one-half ;  and  there  is  a 
still  greater  gain  if  the  bottom  of  the  ves- 
sel is  rounded  outward  to  meet  the  tube, 
as  at  B  in  Fig.  165. 

If,  however,  the  tube  extends  into  the 


Fig.  164.        Fig.  165.         Fig.  166.' 


A 

is 


vessel,  as  at  C  in  Fig.  166,  instead  of  increasing  the  discharge,  it  obstructs 
and  diminishes  it. 

365.      FLOW    OF    LIQUIDS    IN    PIPES    AND   THE    BEDS     OF 

STREAMS. — The  friction  of  water  against  the  sides  of  pipes 
in  which  it  is  conveyed,  retards  its  velocity  and  diminishes 
the  quantity  discharged. 

When  the  distance  is  great,  or  there  are  sudden  turn- 
ings, allowance  must  be  made  for  friction  by  increasing  the 
size  of  the  pipes,  or  the  quantity  discharged  will  fall  far 
below  what  is  required.  If,  for  instance,  leaving  friction 
out  of  account,  pipes  6  inches  in  diameter  would  yield  the 
desired  supply,  nine-inch  pipes  would  be  none  too  large  to 
use. 

example.  362.  With  a  given  area,  what  shape  must  an  orifice  have,  to  discharge  the 
most  liquid  ?  Why  is  this  ?  363.  How  may  the  volume  discharged  be  increased  ? 
864.  What  other  mode  of  increasing  the  discharge  is  there  ?  Describe  the  kinds  of 
adjutage  mentioned  in  the  text,  and  state  the  effect  of  each.  365.  What  is  the  effect 
of  friction  on  the  flow  of  liquids  ?  How  great  an  allowance  should  be  made  for  fric- 


156  HYDRAULICS. 

366.  Rivers. — The  friction  of  a  stream  against  its  banks 
and  bottom  materially  retards  its  motion.  Hence  the  ve- 
locity of  a  river  is  always  less  near  its  banks  than  towards 
the  centre,  and  near  the  bottom  than  at  the  surface. 

The  windings  of  a  stream  also  lessen  its  velocity.  Were 
it  not  for  their  numerous  bends,  many  large  rivers  would 
flow  so  rapidly  that  they  could  not  be  navigated. 

367.  The  velocity  of  a  stream  depends  much  on  the  slope  of  its  bed.  A 
river  with  but  few  bends,  and  a  fall  of  three  inches  to  the  mile,  moves  at  the 
rate  of  about  three  miles  an  hour.  As  the  slope  increases,  the  velocity  rap- 
idly increases  also ;  and  a  fall  of  three  feet  in  a  mile  gives  the  impetuosity  ot 
a  torrent. 

Sometimes  the  bed  of  a  river  has  a  considerable  fall  at  first,  and  then  be- 
comes comparatively  level.  In  such  cases,  the  impetus  of  the  water  keeps 
it  in  motion  at  a  rate  proportioned  to  its  volume.  The  fall  of  the  Amazon, 
in  the  last  700  miles  of  its  course,  is  only  12  feet. 

368.  The  quantity  of  water  discharged  by  a  stream  de- 
pends on  its  size  and  velocity.     In  large  rivers,  it  is  almost 
incredible.    The  discharge  of  the  Mississippi  is  estimated 
at  twelve  billions  of  cubic  feet  every  minute,  and  that  of 
the  Amazon  is  nearly  four  times  as  great. 

369.  Waves. — Waves  are  caused  by  the  action  of  wind 
on  a  liquid  surface.     As  the  particles  of  a  liquid  move  freely 
among  each  other,  the  undulations  produced  directly  by 
the  wind  extend  along  the  surface  to  a  great  distance,  far- 
ther than  the  wind  itself. 

The  wind  is  enabled  to  take  hold,  as  it  were,  of  the  water,  and  produce 
waves,  by  the  friction  at  the  surface.  This  friction  may  be  diminished,  just 
as  in  the  case  of  machinery,  by  covering  the  surface  with  oil.  The  wind 
then  slips  over  it,  and  the  water  becomes  comparatively  calm.  It  is  said 
that  boats  have  been  enabled  to  get  through  a  dangerous  surf  in  safety,  by 
emptying  barrels  of  oil  upon  it. 

370.  Waves  appear  to  move  forward,  but  in  deep  water  they  only  move 


tion  ?  306.  "Where  has  the  water  of  a  river  the  least  velocity,  and  why  ?  What  effect 
have  the  windings  of  a  stream  on  its  velocity  ?  36T.  On  what  does  the  velocity  of  a 
stream  chiefly  depend  ?  How  great  a  velocity  does  a  fall  of  three  inches  in  a  mile 
produce  ?  How  great  a  fall  produces  the  velocity  of  a  torrent  ?  How  great  a  fall  has 
the  bed  of  the  Amazon  near  its  mouth  ?  What  keeps  its  waters  in  motion  ?  368.  On 
what  does  the  quantity  of  water  discharged  by  a  stream  depend  ?  How  great  is  the 
discharge  of  the  Mississippi?  Of  the  Amazon?  869.  By  what  are  waves  caused? 
What  enables  the  wind  to  produce  waves?  How  may  a  rough  sea  be  calmed? 


WAVES.  157 

up  and  down.  A  floating  body,  after  rising  and  falling  with  successive  waves, 
when  the  sea  becomes  calm  is  found  in  the  same  spot  as  before.  If,  how- 
ever, shoals  or  rocks  interfere  with  the  undulations,  an  onward  motion  is 
prodaced,  and  breakers  are  formed.  Waves  are  always  found  breaking  on  a 
rocky  shore,  whatever  way  the  wind  may  blow. 

371.  Waves  do  not  generally  exceed  20  feet  in  height, 
— that  is,  do  not  rise  more  than  10  feet  above,  and  sink 
more  than  10  feet  below,  the  usual  level  of  the  sea.    They 
sometimes,  however,  attain  a  height  of  40  feet.     Vast  and 
mighty  as  they  are,  their  effects  are  confined  to  the  surface, 
never  extending  to  the  great  body  of  the  ocean.     The  se- 
verest gales  are  not  felt  at  a  depth  of  200  feet. 

372.  Tides. — In  the  ocean,  and  the  bays,  rivers,  &c., 
communicating  with  it,  there  is  an  alternate  rise  and  fall 
of  wajfcer,  each  lasting  about  six  hours.     These  movements 
are  called  Tides.    When  rising,  the  tide  is  said  to  flow  • 
when  falling,  to  ebb. 

373.  Tides  are  caused  chiefly  by  the  attraction  of  the  moon.  This  body, 
when  opposite  any  given  part  of  the  earth,  attracts  the  water  at  that  part 
most  strongly  towards  itself,  and  causes  high  tide.  At  the  same  time  it  is 
high  tide  at  the  opposite  point  of  the  globe,  because  the  moon,  attracting  the 
mass  of  the  earth  more  strongly  than  the  more  distant  water  on  its  surface, 
draws  the  former,  as  it  were,  away  from  the  latter.  These  elevations  are 
accompanied  with  corresponding  depressions,  or  low  tides,  at  other  points. 

The  sun,  also,  attracts  the  water  on  the  earth's  surface ;  but  not  so  strongly 
as  the  moon,  in  consequence  of  its  vast  distance.  When  sun  and  moon  act 
in  the  same  direction,  which  happens  at  every  new  and  full  moon,  the  tides 
are  highest,  and  are  called  Spring-tides.  When  sun  and  moon  act  in  oppo- 
site directions,  the  tides  are  lowest,  and  are  called  Neap-tides. 

374.  The  height  of  the  tide  is  affected  by  prevailing 
winds,  the  shape  of  adjacent  coasts,  and  other  circum- 
stances ;  accordingly,  it  is  different  in  different  places.  At 
St.  Helena,  the  rise  of  water  is  only  3  feet ;  in  parts  of  the 
British  Channel,  it  is  60.  The  highest  tides  known  are  in 
the  Bay  of  Fundy,  where  they  attain  a  height  of  70  feet. 

870.  Ho-w  do  waves  appear  to  move  ?  How  do  they  really  move  ?  What  proof  is 
there  of  this  ?  What  is  the  effect  of  shoals  or  rocks  ?  871.  What  is  the  height  of 
waves  ?  How  far  below  the  surface  do  they  extend  ?  372.  What  are  Tides  ?  873.  By 
what  are  tides  caused?  What,  besides  the  moon,  attracts  the  water?  What  are 
Spring-tides,  and  how  are  they  caused  ?  What  are  Neap-tides,  and  when  do  they 
occur  ?  874.  By  what  circumstances  is  the  height  of  tides  affected  ?  How  great  is 


158 


HYDRAULICS. 


Fig.  167. 


This  makes  the  average  rise  one  foot  every  five  minutes, — 
so  rapid  a  flow  that  animals  feeding  on  the  shore  are  some- 
times overtaken  and  drowned. 

375.  WATER-WHEELS. — Running  water  is  exceedingly 
useful  as  a  moving  power.  Made  to  act  on  wheels,  it  causes 
them  to  revolve  by  its  momentum,  turns  the  shafts  or  axles 
connected  with  them,  and  thus  sets  machinery  of  various 
kinds  in  motion. 

The  wheels  moved  by  water-power  are  of  four  kinds ; 
the  Undershot,  the  Overshot,  the  Breast-wheel,  and  the 
Turbine. 

376.  THE  UNDERSHOT  WHEEL 
is  represented  in  Fig.  167.  A 
wheel,  A  B,  attached  to  an  axle, 
0,  has  a  number  of  Jloqf-boards, 
c,  d,  e,f,  fitted  into  its  rim,  at 
right  angles,  and  at  equal  dis- 
tances from  each  other.  The 
whole  is  hung  in  such  a  way  that 
the  lowest  float-board,  c,  is  im- 
mersed in  running  water,  M  N. 
The  current,  striking  against 
several  float-boards,  which  are 
more  or  less  submerged,  carries 
the  wheel  around. 

The  stream  is  often  conduct- 
ed to  the  wheel  through  a  nar- 
row passage  called  a  Race  /  and 
its  power  is  sometimes  increased  by  giving  the  race  a  slight  inclination  (see 
Figure).  In  other  cases,  the  water  is  made  to  strike  the  wheel  immediately 
after  issuing  from  the  bottom  of  a  dam,  with  a  high  degree  of  velocity  pro- 
duced by  the  pressure  of  a  large  body  of  water.  Yet,  under  the  most  favor- 
able circumstances,  as  the  weight  of  the  water  does  not  act  on  the  wheel,  but 
only  the  force  of  the  current,  no  more  than  one-fourth  of  the  moving  power 
can  be  made  available. 

377.  THE  OVERSHOT  WHEEL  is  represented  in  Fig.  168.  It  consists  of  a 
wheel,  A  B,  attached  to  an  axle,  0,  and  having  a  number  of  "buckets,  c,  d,  e,f, 
on  its  rim,  at  equal  distances.  A  stream  is  conducted  through  a  race,  G  H, 


THE  TTNDEBSHOT  WHEEL. 


the  rise  at  St.  Helena  ?  In  the  British  Channel  ?  In  the  Bay  of  Fundy  ?  3T5.  How 
is  running  water  turned  to  account?  Name  the  four  kinds  of  water-wheels.  376.  De- 
scribe the  Undershot  Wheel.  How  is  the  stream  often  conducted  to  the  wheel? 
How  is  its  power  increased  ?  In  other  cases,  how  is  a  high  degree  of  velocity  pro- 
duced? How  much  of  the  moving  power  can  be  made  available  with  this  wheel? 


WATER-WHEELS . 


159 


and  made  to  fall  on  Fig.  IGS. 

the  wheel  from  above. 
The  weight  of  the  wa- 
ter and  the  force  with 
which  it  descends 
cause  the  wheel  to  re- 
volve. Another  buck- 
et is  brought  under 
the  stream,  which  in 
its  turn  is  filled,  and  a 
new  one  is  presented. 

As  the  wheel  turns, 
the  descending  buck- 
ets gradually  lose  their 
water,  so  that  by  the 
time  they  commence 
rising  they  are  entire-  TUB  OVEBSHOT  WHEEL. 

ly  empty.    As  the  de- 
scending buckets  contain  more  or  less  water  and  the  ascending  ones  contain 
none,  the  wheel  is  kept  revolving ;  and  the  weight  of  the  stream,  as  well  as 
its  velocity,  being  turned  to  account,  three-fourths  of  the  moving  power  is 
saved. 

878.  In  THE    BREAST-  Fig.  169. 

WHEEL,  shown  in  Fig.  169, 
there  is  a  similar  arrange- 
ment of  apartments  on  the 
rim.  The  water  is  received 
halfway  up,  or  still  higher 
in  the  High  Breast-wheel 
commonly  used  in  this 
country ;  and  its  weight  is 
thus  made  available.  This 
wheel  ranks  between  the 
Overshot  and  the  Under- 
shot in  efficiency,  saving 
three-fifths  of  the  moving 
power. 

379.  THE  TURBINE,  a 
section  of  which  is  represented  in  Fig.  170,  instead  of  being  vertical,  like  the 
wheels  just  described,  is  horizontal.  It  consists  of  a  wheel,  A  B,  divided  into 
a  number  of  apartments,  c,  d,  e,f,  by  curved  partitions.  To  the  inner  rim 
of  the  wheel  is  fitted  a  fixed  cylinder,  G  H,  divided  into  apartments  corre- 
sponding with  those  of  the  wheel,  but  running  in  the  opposite  direction. 

37T.  Describe  the  Overshot  Wheel.  Explain  its  operation.  How  much  of  the  moving 
power  does  it  utilize  ?  3T8.  In  the  Breast-wheel,  how  is  the  water  received  ?  How 
much  of  the  moving  power  is  utilized  ?  879.  Describe  the  Turbine.  Explain  it§ 


THE  BREAST-WHEKL. 


160  HYDE  AULICS. 

Fig.  1TO.  This  fixed  cylinder  is  connected  with  the 

base  of  an  upright  tube,  J  K,  through  the 
middle  of  which  runs  another  tube,  I. 

The  water  which  is  to  set  the  machinery 
in  motion  enters  J  K,  runs  through  the 
apartments  of  G  H,  is  discharged  by  them 
into  the  corresponding  apartments  of  the 
wheel,  and  passes  out  into  a  course  pro- 
vided for  its  escape.  It  strikes  the  parti- 
tions nearly  at  right  angles,  and  with  great 
force  in  consequence  of  the  pressure  of 
the  liquid  in  the  tube.  The  wheel  is  thus 
made  to  revolve ;  and  a  shaft  connected 

THE  TUBBINE.  ...      .,     ,,  ,     ,  ,  .  ,,  , 

with  it  from  below  and  passing  through 

the  inner  tube  I,  communicates  the  motion  to  machinery  above.  Wherever 
there  is  a  fall  of  water,  turbines  are  found  very*useful.  They  have  been 
known  to  utilize,  or  turn  to  account,  four-fifths  of  the  motive  power, — more 
than  is  saved  by  any  other  wheel. 

380.  PROPULSION  OF  BOATS. — The  wheels  of  steamboats 
are  not  turned  by  running  water,  like  those  described  above, 
but  by  machinery  worked  by  steam.     As  they  strike  the 
water,  the  latter  reacts  on  them ;  and  the  boats  are  forced 
forward  or  backward,  according  to  the  direction  in  which 
their  wheels  turn.     Paddles  on  their  rim  enable  the  wheels 
to  strike  the  water  more  forcibly. 

As  the  paddles  descend  and  ascend,  they  have  to  over- 
come a  considerable  resistance  in  a  vertical  direction,  which 
retards  their  motion ;  it  is  only  when  they  are  vertical  in 
the  water  that  their  full  effect  is  felt.  The  rolling  of  the 
boat,  also,  often  interferes  with  their  action,  burying  them 
too  deep  x>r  raising  them  entirely  out  of  water.  These  dis- 
advantages have  led  some  to  prefer  a  submerged  screw  to 
the  paddle-wheel.  The  screw  is  placed  in  the  stern  ;  and 
vessels  moved  by  its  means  are  called  Screw  Propellers. 

381.  The  resistance  which  a  vessel  encounters  in  passing 
through  water  depends  on  its  shape.     The  narrower  the 
vessel  and  the  sharper  its  prow,  the  more  readily  it  pene- 

operation.  How  much  of  the  moving  power  have  turbines  been  known  to  utilize  ? 
380.  How  are  steamboats  moved  ?  What  disadvantage  do  the  paddles  labor  under  ? 
What  is  substituted  in  some  vessels  for  the  paddle-wheel  ?  What  is  a  vessel  moved 
by  a  screw  called  ?  381.  On  what  does  the  resistance  a  moving  vessel  encounters  from 


BARKER'S  MILL. 


161 


Fig.  in. 


trates  the  water,  on  the  principle  of  the  wedge.  Too  great 
narrowness,  on  the  other  hand,  is  dangerous  in  boats  that 
navigate  stormy  waters,  and  does  not  allow  sufficient  room 
for  freight.  To  determine  the  shape  that  best  combines 
speed,  safety,  and  capacity,  is  the  work  of  the  ship-builder. 
It  is  a  difficult  problem,  and  one  that  is  perhaps  not  yet 
solved,  though  great  advances  have  been  made  of  late  years 
in  naval  architecture. 

382.  BARKER'S  MILL. — An  ingenious  hydraulic  machine, 
called  Barker's  Mill,  and  represented  in  Fig.  171,  remains 
to  be  described. 

A  is  an  upright  hollow  cylinder,  turning  freely 
on  a  vertical  axis.  Through  its  lower  end  runs  a 
horizontal  tube,  B  C,  communicating  internally  with 
the  cylinder.  On  opposite  sides  of  this  tube,  at  its 
extremities,  are  two  small  openings.  A  continuous 
stream  is  introduced,  through  the  pipe  D  E,  into  the 
funnel  at  the  top  of  the  cylinder  A.  It  runs  down 
into  the  cross-tube  B  C  ;  and,  if  there  were  no  op- 
portunity of  escape,  it  would  there  rest,  pressing 
equally  in  every  direction.  The  moment,  however, 
that  the  two  holes  in  the  ends  are  opened,  the  wa- 
ter runs  through ;  and  the  pressure  at  the  holes  be- 
ing thus  removed,  while  that  on  the  opposite  sides 
remains  undiminished,  the  tube  is  forced  round  in 
the  direction  of  the  pressure,  that  is,  in  an  opposite 
direction  to  the  jets  of  water.  The  cylinder  A  turns 
with  the  tube,  and  thus  motion  is  communicated  to 
the  mill-stone  S.  H  is  a  hopper,  which  feeds  the 
mill  with  grain. 

383.  MACHINES  FOR  RAISING  WATER. 


BARKER  8   MILL. 


— It  is  often  desirable  to  raise  water  from  a  lower  to  a 
higher  level.  Well-sweeps,  acting  on  the  principle  of  the 
lever,  are  used  for  this  purpose,  as  is  also  the  wheel  and 
axle  in  a  variety  of  forms.  But,  when  a  large  supply  is  re- 
quired, other  machines,  worked  with  less  expense  of  time 
and  labor,  are  employed.  Some  of  these  involve  the  prin- 
ciples of  Pneumatics,  and  will  be  treated  under  that  head. 

the  water  depend  ?  What  is  the  advantage,  and  what  the  disadvantage,  of  narrow- 
ness and  sharpness  of  prow?  882.  Describe  Barker's  Mill,  and  its  mode  of  operation. 
383.  What  machines  are  used  for  raising  water  ?  884.  What  is  one  of  ttie  simplest 


162 


HYDRAULICS. 


Fig.  172. 


Those  that  belong  exclusively  to  Hydraulics  are  described 
below. 

384.  Archimedes*  Screw. — The  Screw  of  Archimedes, 
called  after  the  philosopher  that  invented  it,  is  one  of  the 
simplest  machines  for  raising  water.     It  consists  of  a  tube 
wound  spirally  round  a  solid  cylinder,  as  represented  in 
Fig.  172. 

To  work  the  machine,  let 
one  end  of  the  tube,  C,  rest 
just  below  the  surface  of  the 
water.  The  cylinder,  AB, 
must  be  inclined  at  an  angle 
of  about  35  degrees,  and  be 
fastened  at  the  lower  end  in 
such  a  way  as  to  revolve 
freely  when  turned  by  the 
handle,  H.  When  the  cylin- 
der is  turned,  the  open  end 
of  the  tube,  C,  scoops  up 
some  of  the  water.  When 

ABCHIMEDES'  SCREW.  it  has  got  half  way  round, 

the  point  D  is  lower  than  the 

end  C,  and  the  water  descends  to  D  by  the  force  of  gravity.  Another  half- 
revolution  brings  the  point  E  lower  than  D,  and  again  the  water  descends. 
This  is  continued  till  the  water  is  discharged  at  the  upper  end.  As  new  wa- 
ter is  constantly  scooped  up,  there  will  be  a  continuous  flow  as  long  as  the 
handle  is  turned. — Archimedes'  Screw  operates  only  at  short  distances. 

385.  The  Chain  Pump. — The   Chain  Pump  is  much 
used  for  raising  water.     The  principle  it  involves  is  also 
applied  in  dredging-machines,  for  cleaning  out  the  channels 
of  rivers. 

This  machine  (see  Fig.  173)  consists  of  a  continuous  chain,  to  which  cir- 
cular plates,  c,  d,  e,f,  Ac.,  are  attached  at  equal  distances.  The  plates  are 
of  such  a  size  as  exactly  to  fit  the  cylinder  G  H,  the  lower  end  of  which  rests 
in  the  water.  The  chain  passes  over  the  two  wheels,  I,  J  ;  to  the  upper  one 
of  which,  I,  a  handle  is  attached.  When  the  handle  is  turned,  the  chain  is 
set  in  motion.  The  plates,  ascending  through  G  H,  carry  up  water  before 
them,  which  has  no  opportunity  of  escaping  till  it  reaches  the  opening  K. 


machines  for  raising  water  ?  Of  what  does  Archimedes'  Screw  consist  ?  Describe  its 
tnode  of  operation.  At  what  distances  does  Archimedes'  screw  operate  ?  3S5.  What 
machine  is  much  used  for  raising  water  ?  What  other  application  is  made  of  the 
principle  Jt  involves?  Describe  the  Chain  Pump,  and  its  mode  of  operating. 


THE   HYDRAULIC   KAM. 


163 


There  it  is  discharged,  as  long  as  the  Fig.  173. 

handle  is  turned. 

386.  The  Hydraulic  Earn. 
— The  Hydraulic  Ram  was  in- 
vented in  France,  in  1796.  It 
raises  water  by  successive  im- 
pulses, which  have  been  com- 
pared to  the  butting  of  a  ram, 
and  hence  its  name.  The  re- 
quisite power  is  gained  by  mo- 
mentarily stopping  a  stream  in 
its  course,  and  causing  its  mo- 
mentum to  act  in  an  upward 
direction. 

Fig.  174  represents  a  simple  form  of 
the  Hydraulic  Ram.  To  a  stream  or  res- 
ervoir at  A,  is  adapted  an  inclined  pipe, 
B,  through  which  the  water  that  works 
the  ram  is  conveyed.  Near  the  lower 
end  of  the  pipe  B  rises  an  air-chamber, 
D,  with  which  an  upright  pipe,  F,  is  con- 
nected. The  passage  connecting  B  with 
the  air-chamber  is  commanded  by  a  valve 
opening  upward.  At  the  extremity  of 

the  pipe  B  is  another  valve,  E,  opening  THK  CHAIN  PirMp. 

downward,  and  made  just  heavy  enough 
to  fall  when  the  water  in  B  is  at  rest. 

Fig.  174.  The  play  of  the  valve  E  makes  the  machine  self- 

acting.    Suppose  the  pipe  B  to  be  filled  from  the  res- 
ervoir ;  the  valve  E  opens  by  its  weight,  and  allows 
some  of  the  water  to  escape.    Soon,  however,  the 
water  acquires  momen- 
tum enough  to  raise  the 
valve  and  close  the  open- 
ing.   The  stream  is  thus 
suddenly  stopped,    and 
the  pipe  would   be   in 
danger  of  bursting  from 
THE  HYDRAULIC  RAM.  the  shock  were  it  not  for 

the  valve  in  the  air-chamber  D,  which  is  at  once  forced  upward,  and  allows 


tS6.  When  and  where  was  the  Hydraulic  Earn  invented  ?     Why  is  it  so  called  ? 
How  is  the  requisite  power  gained  in  the  ram  ?    Describe  the  hydraulic  ram,  and 


164  H  YDBAUUCS. 

some  of  the  water  to  enter.  The  air  in  D  is  at  first  condensed  by  the  pressure 
of  the  water  thus  admitted  ;  but,  immediately  expanding  by  reason  of  its  elas- 
ticity, it  drives  the  water  into  F,  for  the  closing  of  the  valve  prevents  it  from 
returning  to  B.  By  this  time  the  water  in  B  is  again  at  rest,  the  valve  E 
opens,  and  the  whole  process  is  repeated. 

By  successive  impulses  the  water  may  be  raised  in  F  to  a  great  height.  A 
descent  of  four  or  five  feet  from  the  reservoir  is  sufficient.  Care  must  be 
taken  to  have  the  valve  E  just  heavy  enough  to  fall  when  B  is  at  rest,  and 
not  so  heavy  as  to  prevent  it  from  readily  rising  as  the  momentum  of  the 
stream  increases.  The  pipe  B  must  also  be  of  such  length  that  the  water, 
when  arrested  in  its  course,  may  not  be  thrown  back  on  the  reservoir. 

387.  Hydraulic  Rams  afford  a  cheap  and  convenient 
means  of  raising  water  in  small  quantities  to  great  heights, 
wherever  there  is  a  spring  or  brook  having  a  slight  eleva- 
tion. They  are  used  for  a  variety  of  purposes,  and  partic- 
ularly when  a  supply  of  water  is  needed  for  agricultural 
operations. 

EXAMPLES  FOR  PRACTICE. 

p5f*  Friction  is  left  out  of  account  in  these  examples. 

1.  (See  §356,  rule  in  italics.)  Two  streams  issue  from  different  orifices  in  the 

same  vessel  with  velocities  that  are  to  each  other  as  1  to  6.  How  many 
times  farther  from  the  surface  is  the  one  than  the  other  ? 

2.  The  stream  A  runs  from  an  orifice  in  a  vessel  three  times  as  fast  as  the 

stream  B.  How  do  their  distances  below  the  surface  of  the  liquid  com- 
pare? 

3.  In  a  vat  full  of  beer  there  are  two  orifices  of  equal  size ;  one  9  inches  be- 

low the  surface,  and  the  other  25.  How  does  the  velocity  of  the  latter 
compare  with  that  of  the  former? 

4.  There  are  three  apertures  in  a  reservoir  of  water,  1,  4,  and  16  feet  below 

the  surface.    With  what  comparative  velocity  will  their  streams  flow  ? 

5.  A  stream  flows  from  an  aperture  in  a  vessel  at  the  rate  of  4  feet  in  a  sec- 

ond. I  wish  to  have  another  stream  from  the  same  vessel  with  a  velo- 
city of  16  feet  per  second.  How  much  farther  below  the  surface  than  the 
first  must  it  be? 

6.  (See  §  359.)  A  vat  full  of  ale,  3  feet  high,  has  four  apertures  in  it,  3,  12, 18, 

and  24  inches  respectively  from  the  top.  Through  which  will  the  liquid 
spout  to  the  greatest  horizontal  distance  ?  Which  next  ?  Which  next  ? 

7.  (See  §  360.)  How  much  water  will  be  discharged  every  minute   from  an 

orifice  of  3  square  inches,  the  stream  flowing  at  the  rate  of  5  feet  in  a 
second,  and  the  vessel  being  kept  replenished  ? 

tts  mode  of  operating.    How  great  a  descent  is  required  ?    What  precautions  are 
pecessary  ?   887.  In  what  case  may  hydraulic  rams  be  used  with  advantage  ? 


EXAMPLES  FOR    PRACTICE.  165 

How  much  will  be  discharged  every  minute  from  another  orifice  in 
the  same  vessel,  equally  large,  but  situated  four  times  as  far  below  the 
surface  of  the  liquid  ? 

A  stream  flows  from  a  hole  in  the  bottom  of  a  vessel  with  a  velocity  of  6 
feet  in  a  second.  The  hole  has  an  area  of  5  square  inches,  and  the  ves- 
sel is  emptied  in  15  seconds.  How  much  water  does  the  vessel  hold  ? 

<See  §  376.)  A  stream  having  a  momentum  equivalent  to  100  units  of  work 
is  applied  to  an  Undershot  Wheel ;  how  many  units  of  work  will  it  per- 
form?—Ans.  25. 

(See  §  377.)  How  many  units  of  work  will  it  perform,  if  applied  to  an  Over- 
shot Wheel? 

(See  §  378.)  How  many,  if  applied  to  a  Breast-wheel? 

(See  §  379.)  How  many,  if  applied  to  a  Turbine? 


CHAPTER  XII. 

PNEUMATICS. 

388.  PNEUMATICS  is  the  science  that  treats  of  air  and  the 
other  elastic  fluids,  their  properties,  and  the  machines  in 
which  they  are  applied. 

389.  DIVISION  OF  ELASTIC  FLUIDS. — The   elastic  fluids 
are  divided  into  two  classes  : — 

I.  GASES,  or  such  as  retain  their  elastic  form  under  ordi- 
nary circumstances.  Some  of  the  gases,  under  a 
high  degree  of  pressure,  assume  a  liquid  form  ;  as, 
carbonic  acid  and  chlorine ;  others,  such  as  oxy- 
gen and  nitrogen,  can  not  be  converted  into  liquids 
by  any  known  process. 

II.  VAPORS,  or  elastic  fluids  produced  by  heat  from 
li quids  and  solids.  When  cooled  down,  they  re- 
sume the  liquid  or  solid  form.  Steam,  the  vapor 
of  water,  is  an  example. 

390.  All  gases  and  vapors  have  the  same  properties. 

888.  "What  is  Pneumatics  ?    389.  Into  what  two  classes  are  elastic  fluids  divided  ? 
What  are  gases  ?    What  difference  is  there  in  the  gases  ?   What  are  vapors  ?    890.  In 


166 


PNEUMATICS. 


The  principles  of  Pneumatics,  therefore,  relate  to  all  alike, 
though  they  are  most  frequently  exhibited  and  applied  in 
the  case  of  air,  with  which  we  have  far  more  to  do  than 
with  any  other  elastic  fluid. 

Air. 

391.  Air  is  the  elastic  fluid  that  we  breathe.    It  sur- 
rounds the  earth  to  a  distance  of  about  fifty  miles  from  its 
surface,  and  forms  what  is  called  the  Atmosphere.     It  exists 
in  every  substance,  entering  the  minutest  pores. 

392.  VACUUMS.  —  Air  may  be  removed  from  a  vessel  with 
an  instrument  called  the  Air-pump.    A  Vacuum  is  then  said 
to  be  produced.     Vacuums  sometimes  result  from  natural 
causes  ;  but  they  last  only  for  an  instant,  as  the  surround- 
ing air  at  once  rushes  in  to  fill  them.     Hence  the  old  phi- 
losophers used  to  say,  Nature  abhors  a  vacuum. 

393.  PROPERTIES  OF  AIR.  —  Air  can  not  be  seen,  but  it 

can  be  felt  by  moving  the  hand 
rapidly  through  it.  It  is  there- 
fore material,  and  has  all  the 
essential  properties  of  matter. 

394.  Air  is  impenetrable. 
395.  The  Dimng-lell.—  The  impen- 
etrability of  air  is  shown  by  the  Diving- 
bell,  represented  in  Fig.  175.  A  C  is  a 
large  iron  vessel,  shaped  somewhat  like 
an  inverted  tumbler,  and  attached  to  a 
chain,  by  which  it  is  let  down  in  the 
water.  As  the  vessel  descends,  the  air 
in  it  is  condensed  by  the  upward  pres- 
sure  of  the  liquid,  and  water  enters. 
The  lower  it  gets,  the  more  the  air 
is  compressed,  and  the  greater  the 
amount  of  water  admitted.  The  im- 
penetrability of  the  air,  however, 
keeps  the  greater  part  of  the  bell 


rig.  175. 


THE   DIVING-BELL. 


what  are  the  principles  of  Pneumatics  most  frequently  exhibited,  and  why? 
891.  What  is  Air  ?  How  far  does  it  extend  from  the  earth's  surface  ?  What  does  it 
constitute  ?  892.  What  is  a  Vacuum  ?  What  did  the  old  philosophers  say,  and  why  ? 
893.  What  proves  the  air  to  be  material  ?  894.  What  apparatus  shows  the  impenetra- 
bility of  air?  895.  Describe  the  Diving-bell.  Explain  how  descents  are  made  with 


PKOPEKTLES   OF  AIR. 


167 


clear  of  water,  so  that  several  persons  may  descend  in  it  to  the  bottom  of 
the  sea. 

As  fast  as  the  air  is  vitiated  by  the  breath,  it  is  let  off  by  a  stop-cock, 
while  fresh  air  is  supplied  from  above  by  a  condensing  syringe,  through  the 
pipe  B.  Air  may  be  thus  forced  down  in  sufficient  quantities  to  expel  the 
water  altogether  from  the  bell,  so  that  the  divers  can  move  about  without 
difficulty  on  the  bottom  of  the  sea.  If  air  were  not  impenetrable,  the  bell 
would  be  filled  with  water,  and  the  divers  drowned. 

When  the  diving-bell  was  invented,  is  not  known.  History  makes  no 
mention  of  it  before  the  sixteenth  century.  At  that  time,  we  are  told,  two 
Greeks,  in  the  presence  of  the  emperor  Charles  V.  and  several  thousand  spec- 
tators, let  themselves  down  under  water,  at  Toledo  in  Spain,  in  a  large  in- 
verted kettle,  and  rose  again  without  being  wet.  In  1665,  a  kind  of  bell  was 
employed  off  the  Hebrides,  for  the  purpose  of  recovering  the  treasure  lost 
in  several  ships  belonging  to  the  Invincible  Armada.  From  that  time  to  the 
present,  various  improvements  have  been  made  in  the  diving-bell ;  and  it  is 
now  extensively  used  for  clearing  out  harbors,  laying  the  foundation  of  sub- 
marine walls,  and  recovering  articles  lost  by  shipwreck. 

396.  Air  is  compressible. 

This  is  proved  with  the  diving-bell.   If  the  air      F1s- 176- 
were  not  compressible,  no  water  would  enter  the 
bell  as  it  descended. 

39V.  Air  is  elastic. 

This  also  may  be  shown  with  the  diving- 
bell.  When,  on  its  descent,  water  has  entered, 
on  account  of  the  air's  being  compressed,  let  the 
bell  be  raised,  and  the  air  will  resume  its  origi- 
nal bulk,  expelling  the  water. 

Bottle  Imps. — The  compressibility  and  elasticity  of  air  may 
be  exhibited  in  an  amusing  way  with  the  apparatus  represent- 
ed in  Fig.  176.  In  a  vessel  nearly  full  of  water  are  placed  sev- 
eral small  balloons,  or  hollow  figures  of  men,  &c.,  made  of  col- 
ored glass,  and  called  Bottle  Imps.  Each  figure  has  a  little 
hole  in  the  bottom,  and  is  of  such  specific  gravity  that  it  will 
just  float  in  water.  A  piece  of  thin  india  rubber  is  tied  over 
the  mouth  of  the  vessel,  so  as  to  cut  off  communication  with 
the  external  air.  Now  press  on  the  india  rubber  cover.  The  BOTTLE  IMPf- 
water  at  once  transmits  the  pressure  to  the  air  in  the  hollow  figures.  This 
air  is  condensed,  water  enters,  the  specific  gravity  of  the  figures  is  increased, 

it. .  What  is  the  first  mention  made  of  the  diving-bell  in  history  ?  In  1665,  for  what 
purpose  was  it  used  ?  For  what  is  it  now  extensively  used  ?  396.  How  does  the 
diving-bell  prove  air  to  be  compressible?  39T.  How  does  it  prove  air  to  be  elastic? 
What  properties  in  air  do  the  Bottle  Imps  illustrate  ?  Describe  the  bottle  imps,  and 


168  PNEUMATICS. 

and  they  descend.  On  removing  the  fingers  from  the  cover,  the  air,  by  rea- 
son of  its  elasticity,  resumes  its  original  bulk,  and  the  figures  rise.  By  thus 
playing  on  the  india  rubber,  the  figures  may  be  made  to  dance  up  and  down. 

398.  Mar  lottos  Law. — The  elastic  fluids  are  the  most 
easily  compressed  of  all  substances*  The  greater  the  pres- 
sure to  which  they  are  subjected,  the  less  space  they  occupy , 
and  the  greater  their  density.  A  body  of  air  which  under 
a  certain  pressure  occupies  a  cubic  foot,  under  twice  that 
pressure  will  be  condensed  into  half  a  cubic  foot ;  under 
three  times  that  pressure,  into  one-third  of  a  cubic  foot,  &c. 
This  principle,  variously  stated,  is  called,  from  its  discov- 
erer, Mariotte's  Law. 

The  more  the  elastic  fluids  are  compressed,  the  greater 
is  their  resistance  to  the  pressure.  Hence,  their  elastic  force 
increases  with  their  density. 

399.  The  Air-gun. — By  subjecting  a  body  of  air  to  a  great  pressure,  we 
may  increase  its  elastic  force  sufficiently  to  produce  wonderful  effects.  The 
Air-gun  is  an  example.  It  consists  of  a  strong  metallic  vessel,  into  which 
air  is  forced  till  it  is  in  a  state  of  high  condensation.  The  vessel  is  then  at- 
tached to  a  barrel  like  that  of  an  ordinary  gun,  to  the  bottom  of  which  a  bul- 
let is  fitted.  Pulling  a  trigger  opens  a  valve,  the  condensed  air  rushes  forth, 
and  drives  the  bullet  out  with  great  force. 

One  supply  of  condensed  air  is  sufficient  for  several  discharges,  though 
each  is  weaker  than  the  preceding  one.  The  labor  required  for  condensing 
the  air  prevents  this  instrument  from  being  much  used ;  but  as  it  makes  less 
noise,  when  discharged,  than  the  ordinary  gun,  it  is  sometimes  employed  by 
assassins. 

400.  Air  has  weight. 

Weigh  a  flask  full  of  air,  and  then  weigh  the  same  flask 
with  the  air  exhausted.  The  difference  indicates  the  weight 
of  the  air  contained. 

401.  Experiments  show  the  weight  of  100  cubic  inches  of  air  to  be  about 
30y2  grains.  This  makes  it  828  times  lighter  than  water.  It  has  been  com- 
puted that  the  weight  of  the  whole  atmosphere  surrounding  the  earth  is  equal 
to  that  of  a  globe  of  lead  60  miles  in  diameter. 


«xplain  the  principle  on  which  they  dance  up  and  down.  398.  What  substances  are 
the  most  easily  compressed  ?  What  is  Mariotte's  Law  ?  To  what  is  the  elastic  forco 
of  gases  and  vapors  proportioned  ?  899 .  How  may  a  body  of  air  be  made  to  produce 
wonderful  effects  ?  What  instrument  proves  this  ?  Describe  the  Air-gun,  and  its 
operation.  Why  is  not  the  air-gun  used  more  ?  By  whom  is  it  sometimes  employed  f 
400.  Prove  that  air  has  weight.  401.  What  is  the  weight  of  100  cubic  inches  of  air  ? 


ATMOSPHERIC  PKESSUKE. 


169 


Atmospheric  Pressure. 

402.  The  particles  of  air,  like  those  of  the  other  elastic 
fluids,  mutually  repel  each  other.    The  atmosphere  would 
therefore  spread  out  into  space,  and  become  exceedingly 
rare,  if  it  were  not  for  the  attraction  of  the  earth.     This 
prevents  it  from  extending  more  than  fifty  miles  from  the 
surface,  and  gives  it  weight. 

403.  Since  air  has  weight,  it  exerts  a  pressure  on  all 
terrestrial  bodies.    This  is  known  as  Atmospheric  Pressure. 
The  pressure  on  any  given  body  is  equal  to  the  weight  of 
the  column  of  air  resting  upon  it,  and  therefore       Fig.  ITT. 
varies  according  to  its  size. 

404.  EXPERIMENTS. — The   pressure  of  the 
atmosphere  is  proved  by  experiments. 

Experiment  1. — Take  a  common  syringe,  represented  in 
Fig.  177,  and  let  the  piston,  P,  rest  on  the  bottom^of  the  bar- 
rel. Insert  the  nozzle,  0,  in  a  vessel  of  water,  and  raise  the 
piston.  The  water  enters  through  0,  and  follows  the  piston, 
as  shown  in  the  Figure. 

What  causes  the  water  to  rise?  The  piston,  being  air- 
tight, as  it  is  drawn  up,  leaves  a  vacuum  behind  it ;  and  the 
pressure  of  the  atmosphere  on  the  water  in  the  vessel  drives 
it  into  the  barrel  through  0.  If  the  piston  does  not  fit  the 
barrel  tightly  enough  to  exclude  the  air  above,  no  water 
enters,  because  the  pressure  of  the  air  from  without  is  then 
counterbalanced  by  that  from  within  the  barrel. 

Exp.  2.— Take  a  small  tube,  close  one  end  with  the 
finger,  fill  it  with  water,  and  carefully  invert  it,  as 
shown  in  Fig.  178.  The  water  is  kept  in  the  tube  by 
atmospheric  pressure.  Remove  the  finger,  and  the 
downward  pressure  of  the  atmosphere,  which  was  be- 
fore cut  off,  will  counterbalance  the  upward  pressure, 
and  the  water  will  fall  by  its  own  weight. 

Exp.  3. — Fill  a  wine-glass  with  water,  and  cover  the 
mouth  with  a  piece  of  stiff  paper.  Place  the  hand  over 
the  paper,  and  invert  the  glass.  On  carefully  removing 

What  is  the  weight  of  the  whole  atmosphere  ?  402.  "What  prevents  the  atmosphere 
from  spreading  out  into  space  ?  403.  "What  is  Atmospheric  Pressure  ?  What  causes 
atmospheric  pressure  ?  To  what  is  the  atmospheric  pressure  on  any  given  body 
equal  ?  404.  Describe  the  experiment  with  the  syringe  that  proves  the  pressure  of 
the  atmosphere.  What  will  prevent  the  water  from  rising  in  the  syringe  ?  Describe 


170 


PNEUMATICS. 


the  hand,  the  water  will  be  found  to  remain  in  the  glass,  supported  there  by 
atmospheric  pressure. 

Fig.  1T9.  Exp.  4. — When  we  raise  the  top  board, 

A,  of  a  common  bellows  (see  Fig.  179),  the 
valve  B  in  the  lower  board  opens.  This  is 
because  a  vacuum  is  formed  within  the  bel- 
lows, and  the  atmospheric  pressure  forces 
THE  BELLOWS.  the  valve  up  and  drives  in  a  portion  of  the 

external  air. 

The  same  principle  is  involved  in  the  act  of  breathing.  The  cells  in  the 
lungs  are  expanded  by  muscular  action,  a  vacuum  is  thus  formed,  and  the 
pressure  of  the  atmosphere  drives  in  the  outer  air  through  the  nose  or  mouth. 
In  a  few  seconds  the  muscles  contract,  and  the  same  air,  laden  with  impuri- 
ties received  from  the  blood  in  the  lungs,  is  expelled. 

Fig.  180.  405.  The  Sucker,  a  play-thing  used  by 

boys,  shows  the  force  of  atmospheric  pres- 
sure. It  consists  of  a  circular  piece  of 
leather  with  a  string  attached  to  the  mid- 
dle. The  leather,  being  first. wet  so  that  it 
may' adapt  itself  to  the  surface,  is  pressed 
firmly  upon  a  flat  stone.  The  string  is  then 
gently  pulled,  so  as  to  form  a  vacuum  be* 
tween  the  leather  and  the  stone.  On  this, 
the  atmospheric  pressure  from  above,  not 
being  counterbalanced  from  beneath,  acts 
on  the  leather  with  such  force  that  a  stone 
of  great  weight  may  be  lifted  without  the  sucker's  becom- 
ing detached.  If  a  hole  is  made  in  the  leather,  air  rushes 
in,  the  pressure  from  above  is  counterbalanced,  and  the 
stone  falls  by  its  own  weight. 

When  flies  walk  on  a  ceiling,  their  feet  act  like  suckers.  Vacuums  are 
formed  beneath  them,  and  they  are  sustained  by  atmospheric  pressure.  It  is 
in  the  same  way  that  the  shell-fish  called  limpets  fasten  themselves  to  rocks. 

406.  Supported  by  the  pressure  of  the  atmosphere  below,  while  it  is  cut 
off  from  that  above,  a  liquid  will  not  flow  from  the  tap  of  a  barrel  unless  a 
small  opening  is  made  in  the  top.  As  soon  as  this  is  done,  air  is  admitted, 

the  experiment  with  a  small  tube  that  proves  the  pressure  of  the  atmosphere.  How 
may  water  be  supported  in  a  wine-glass  by  atmospheric  pressure  ?  How  is  the  pres- 
sure of  the  atmosphere  exhibited  with  a  common  bellows?  How  do  we  breathe ? 
405.  Explain  the  principle  involved  in  the  Sucker.  How  are  flies  able  to  walk  on  a 
ceiling?  405.  Why,  when  a  barrel  is  tapped,  must  a  hole  be  made  in  the  top? 


THE  SUCKER. 


THE  BAROMETER.  171 

the  upward  pressure  is  counterbalanced,  and  the  liquid  flows  continuously. 
On  the  same  principle,  a  small  hole  is  made  in  the  lid  of  a  tea-pot. 

407.  THE  BAROMETER. — The  pressure  of  the  atmosphere 
differs  at  different  times  and  different  places.  To  measure 
it,  an  instrument  called  the  Barometer  is  used. 

The  barometer  was  invented  about  the  middle  of  the 
seventeenth  century.  It  was  the  result  of  a  celebrated  ex- 
periment performed  by  Torricelli  \to-re-chel '-le\,  the  friend 
and  pupil  of  Galileo. 

403.  Torricellian  Experiment.— The  Duke  of  Tuscany,  having  dug  a  well 
of  great  depth,  and  tried  to  raise  water  from  it  with  an  ordinary  pump,  found 
to  his  surprise  that  the  water  would  not  rise  more  than  32  feet.  Galileo,  to 
whom  the  fact  was  referred,  was  unable  to  explain  it ;  but  shortly  before  his 
death  he  requested  Torricelli  to  investigate  the  subject.  Torricelli, 
suspecting  that  the  water  was  raised  and  supported  by  atmospheric 
pressure,  proceeded  to  test  the  truth  of  his  opinion  by  experiment- 
ing with  a  column  of  mercury.  Mercury  is  nearly  14  times  as  heavy 
as  water ;  if,  therefore,  atmospheric  pressure  supported  a  column 
of  water  32  feet  high,  it  would  support  a  column  of  mercury  only 
about  one-fourteenth  of  that  height,  or  28  inches.  Accordingly,  he 
procured  a  tube  3  feet  long,  sealed  at  one  end ;  and  having  filled  it 
with  mercury,  and  stopped  the  open  end  with  his  finger,  he  invert- 
ed the  tube  in  a  vessel  of  mercury,  as  shown  in  Fig.  181.  When  he 
removed  his  finger,  the  mercury  fell,  and  finally  settled,  as  he  had 
supposed  it  would,  at  a  height  of  about  28  inches,  leaving  a  vacuum 
in  the  upper  part  of  the  tube.  This  is  the  famous  Torricellian  Vac- 
uum. 

Torricelli  did  not  live  to  follow  up  his  discovery ;  but  the  French 
philosopher,  Pascal,  succeeded  him  with  a  variety  of  ingenious  ex- 
periments. It  occurred  to  Pascal  that,  if  the  columns  of  water  and 
mercury  were  supported  by  the  pressure  of  the  atmosphere,  then 
at  great  elevations,  where  this  pressure  would  necessarily  be  less, 
the  height  of  the  columns  supported  would  also  be  less.  He  tried 
the  experiment  on  a  mountain  in  Auvergne  \p-varn1}.  At  the  foot 
of  the  mountain,  the  mercury  stood  at  28  inches ;  at  the  top,  it  was 
below  25 ;  and  at  intervening  distances  it  stood  between  the  two. 
This  proved  beyond  doubt  that  the  atmosphere  exerted  a  pressure, 
and  that  this  pressure  varied  according  to  the  distance  above  the 
level  of  the  sea.  Perceiving  how  valuable  such  an  instrument  would  be  for 


407.  What  is  the  Barometer  ?    When  was  it  invented  ?    Of  what  was  it  the  result  ? 

408.  Relate  the  circumstances  that  first  directed  attention  to  the  subject.    Give  an 
account  of  Torricelli's  experiment.     What  is  meant  by  the  Torricellian  Yacuum  ? 
Who  followed  up  Torricelli's  discovery  ?    Give  an  account  of  Pascal's  experiment. 


172 


PNEUMATICS. 


Fig.  182. 


measuring  heights,  Pascal  soon  constructed  a  Barometer,  consisting  of  a  tube 
and  vessel  of  mercury  so  attached  as  to  be  conveniently  carried. 

409.  Kinds  of  Barometers. — There  are  several  kinds 
of  barometers.  The  simplest  consists  of  Torricelli's  tube 
and  vessel  of  mercury,  with  a  graduated  scale  attached  to 
the  upper  part.  The  mercury  never  rises  above  31  inches, 
and  seldom  falls  below  27.  The  scale  is  therefore  applied 
only  to  that  part  of  the  tube  which  lies 
between  these  limits. 

The  Wheel  Barometer  is  exhibited 
in  Fig.  182. 

•  Here  the  tube,  instead  of  resting  in  a  vessel  of 
mercury,  is  bent  upward  at  its  lower  extremity. 
A  float,  F,  is  supported  by  the  mercury  in  the  short 
arm  of  the  tube.  To  this  float  is  attached  a  thread, 
which  passes  over  the  pulley  P,  and  is  attached  to 
the  ball  W.  When  the  mercury  falls  in  the  long 
arm  of  the  tube,  it  must  rise  in  the  short  arm,  and 
with  it  rises  the  float  F.  The  thread  turns  the 
pulley  P,  and  this  moves  the  index  I,  which  is  so 
arranged  as  to  traverse  the  graduated  scale  S  S. 

410.  The  Barometer  as  a  Weather- 
guide. — The  barometer  shows  that  the 
pressure  of  the  atmosphere  at  any 
given  place  is  different  at  different 
times.  This  is  because  the  air  is  con- 
stantly varying  in  density,  on  account 
of  a  greater  or  less  intermixture  of  for- 
'eign  substances.  When  the  air  is 
r  densest,  the  mercury  stands  highest, 

and  we  generally  have  clear  weather ; 
but,  when  the  air  is  rarefied,  the  mer- 
THE  WHEEL  BAROMETER     cury  falls,   and  rain  not  unfrequently 
follows.     Hence,  the  barometer  has  been  used  for  predict- 


What  did  it  prove  ?  409.  Of  what  does  the  simplest  kind  of  barometer  consist?  To 
•what  part  of  the  tube  is  the  scale  confined,  and  why  ?  Describe  the  Wheel  Barom- 
eter, and  its  mode  of  operation.  410.  What  does  the  barometer  show  with  respect  to 
the  pressure  of  the  atmosphere  ?  What  occasions  this  difference  ?  When  the  air  is 
densest,  what  generally  follows  ?  When  it  is  rarefied,  what  follows  ?  In  view  of  this, 


THE   BAEOMETEE.  173 

ing  changes  of  weather;  and  the  words  FAIR,  CHANGE,  EAIN, 
are  placed  at  different  points  on  the  scale,  to  indicate  the 
weather  which  may  be  expected  when  the  mercury  reaches 
either  of  those  levels. 

411.  The  only  reliable  indications,  however,  afforded  by  the  barometer 
are  changes  in  the  level  of  the  mercury.  No  regard  should  be  paid  to  the 
particular  point  at  which  it  stands  at  any  given  time  ;  we  should  merely  ask, 
is  it  rising  or  falling  ?  The  following  rules  generally  hold  good : — 

1.  After  much  dry  weather,  if  the  mercury  falls  steadily,  rain  will  ensue, 

though  it  may  not  begin  for  several  days.     The  longer  it  is  in  com- 
ing, the  longer  it  will  last. 

2.  After  much  wet  weather,  if  the  mercury,  standing  below  its  medium 

height,  rises  steadily,  fine  weather  will  ensue,  though  it  may  not  be- 
gin for  several  days.   The  longer  it  is  in  coming,  the  longer  it  will  last. 

3.  A  sudden  fall  of  the  barometer,  in  spring  or  fall,  indicates  wind ;  in 

very  hot  weather,  a  thunder-storm  ;  in  winter,  a  change  of  wind,  and 
rain  or  snow  according  to  the  temperature. 

4.  Sudden  changes  of  the  mercury  indicate  violent  changes  of  the  weather, 

but  not  permanent  ones. 

5.  A  rise  of  mercury  in  autumn,  after  much  wet  and  windy  weather,  indi- 

cates the  approach  of  cold. 

412.  At  sea,  the  barometer  may  be  relied  on  with  tole- 
rable certainty,  and  it  is  therefore  exceedingly  useful  to 
navigators.  Violent  and  frequent  changes  in  the  mercury 
almost  invariably  precede"  a  sudden  storm.  Warned  in 
time,  the  prudent  mariner  furls  his  sails,  and  thus  escapes 
the  fury  of  the  hurricane  which  would  have  proved  fatal  to 
his  craft  had  it  struck  her  unprepared. 

Dr.  Arnott  gives  the  following  account  of  his  preservation  at  sea  through 
the  •«  arning  of  the  barometer : — "  It  was  in  a  southern  latitude  ;  the  sun 
had  just  set  with  placid  appearance,  closing  a  beautiful  afternoon ;  and  the 
usual  mirth  of  the  evening  watch  was  proceeding,  when  the  captain's  order 
came  to  prepare  with  all  haste  for  a  storm  :  the  barometer  had  begun  to  fall 
with  appalling  rapidity.  As  yet  the  oldest  sailors  had  not  perceived  even  a 
threatening  in  the  sky,  and  were  surprised  at  the  extent  and  hurry  of  the 
preparation ;  but  the  required  preparations  were  not  completed,  when  a  more 
awful  hurricane  burst  upon  them  than  the  most  experienced  had  ever  braved. 

to  what  use  has  the  barometer  been  applied  ?  411.  What  are  the  only  reliable  indi- 
cations afforded  by  the  barometer  ?  What  does  a  steady  fall  of  mercury  in  the  ba- 
rometer after  much  dry  weather  indicate  ?  What  does  a  rise  of  mercury  after  much 
wet  weather  indicate  ?  What  does  a  sudden  fall  indicate  at  the  different  seasons  ? 
What  do  sudden  changes  indicate  ?  What  does  a  rise  of  mercury  in  autumn  indicate  ? 
412.  What  is  said  of  the  barometer  at  sea  ?  Eelate  the  circumstances  of  Dr.  Arnott's 


174 


PNEUMATICS. 


Nothing  could  withstand  it ;  the  sails,  already  furled  and  closely  bound  to 
the  yards,  were  riven  away  in  tatters ;  even  the  bare  yards  and  masts  were 
in  great  part  disabled,  and  at  one  time  the  whole  rigging  had  nearly  fallen 
by  the  board.  In  that  awful  night,  but  for  the  little  tube  of  mercury  which 
had  given  the  warning,  neither  the  strength  of  the  noble  ship  nor  the  skill 
and  energies  of  the  commander  could  have  saved  one  man  to  tell  the  tale." 

413.  DENSITY  OF  THE  AIR  AT  DIFFERENT  LEVELS. — The 
lowest  parts  of  the  atmosphere  are  the  densest,  as  they 


Fig.  183. 


1.  Highest  Peak  of  the  Himalayas. 

2.  Highest  Peak  of  the  Alps. 
C.  Highest  Peak  of  the  Andes. 

4.  Mount  Mitchell,  N.  Carolina. 


have  the  greatest  quanti- 
ty of  air  pressing  on  them 
from  above. 

414.  At  the  level  of 
the  sea,  the  pressure  of 
the  atmosphere  on  every 
square  inch  of  surface  is 
15  pounds.    The  body  of 
a  man  of  ordinary  size  has 
a  surface  of  about  2,000 
square  inches,  and  is  there- 
fore subjected  to  the  enor- 
mous pressure  of  30,000 
pounds.     We  do  not  feel 
this  pressure,  because  it  is 
counterbalanced  by  that 
of    the    air   within    our 
bodies. 

415.  The  higher  we  go 
above  the  level  of  the  sea, 
the  less  is  the  pressure  of 
the   atmosphere  and  the 
rarer  the  air.     At  an  ele- 

d  I  vation.  of  18  miles,  the 
mercury  would  fall  to  1 
inch,  —  that  is,  the  air 
above  that  point  is  so  rare, 


preservation  at  sea  by  means  of  the  barometer.  413.  What  parts  of  the  atmosphere 
are  densest,  and  why  ?  414.  How  great  is  the  pressure  of  the  atmosphere  at  the  level 
of  the  sea  ?  How  great  is  the  pressure  on  the  body  of  a  man  of  ordinary  size  ?  Why 


DENSITY   OF  AIR  AT  DIFFEEENT   LEVELS.  1*75 

that  a  column  of  it  30  miles  high  weighs  no  more  than  an 
equal  column  of  mercury  1  inch  in  height. 

The  shading  in  Fig.  183  shows  the  gradual  increase  in  the  density  of  the 
air  as  the  surface  of  the  earth  is  approached.  The  figures  in  the  left  margin 
represent  the  height  of  the  atmosphere  in  miles ;  those  on  the  right,  the  cor- 
responding height,  in  inches,  at  which  the  mercury  stands  in  the  barometer. 
On  the  top  of  Mount  Mitchell  and  Mount  Washington,  the  most  elevated  peaks 
in  the  United  States  east  of  the  Mississippi,  somewhat  over  a  mile  high,  it 
stands  at  24  inches ;  on  the  highest  peaks  of  the  Himalayas  and  Andes,  which 
are  about  five  miles  high,  at  no  more  than  12. 

416.  The  rarity  of  the  air  is  painfully  felt  by  those  who 
ascend  to  great  heights  on  mountains.     The  pressure  of  the 
external  air  being  diminished,  that  which  is  in  the  body 
expands,  the  delicate  blood-vessels  burst,  the  skin  cracks, 
and  blood  issues  from  the  nose  and  ears.   Among  the  Andes, 
the  Indians  are  subject  to  a  malady  called  veta,  which  is 
caused  by  the  rarity  of  the  air.     The  head  aches  violently, 
its  veins  are  swollen,  the  extremities  grow  cold,  and  breath- 
ing becomes  difficult. 

Effect  of  Heat  on  Air. 

417.  Air  is  rarefied  by  heat. 

Throw  some  burning  paper  into  a  wine-glass,  and  before  the  flame  goes 
out  place  your  hand  over  the  top.  The  glass  will  be  found  to  adhere  to  your 
hand.  This  is  because  the  heat  rarefies  the  air  within,  and  thus  expels  most 
of  it  before  the  top  is  covered.  The  pressure  of  the  external  air,  not  being 
counterbalanced  by  any  pressure  from  within,  fastens  the  glass  and  hand 
together. 

418.  Cupping-glasses  are  made  to  draw  on  this  principle.  Incisions  hav- 
ing been  made  in  the  skin,  the  sides  of  the  glass  are  moistened  with  alcohol, 
and  flame  is  applied.  While  the  alcohol  is  burning,  the  glass  is  inverted  on 
the  skin.  The  pressure  of  the  air  in  the  body,  no  longer  counterbalanced  by 
the  external  pressure,  causes  a  flow  of  blood  into  the  cup. 

419.  Heated  air,  being  lighter  than  that  which  surrounds 


do  we  not  feel  this  pressure  ?  415.  What  is  said  of  the  air,  as  we  ascend  above  the 
*ea-level  ?  How  would  the  mercury  stand  at  a  height  of  18  miles?  What  does  Fig. 
183  show  ?  How  does  the  mercury  in  the  barometer  stand  on  the  top  of  Mount 
Mitchell  ?  On  the  tops  of  the  Himalayas  ?  416.  What  sensations  are  experienced 
by  persons  who  ascend  to  great  heights  on  mountains  ?  Describe  the  symptoms  of 
the  neta.  417.  What  is  the  effect  of  heat  on  air  ?  How  may  the  rarefaction  of  air 
by  heat  be  shown  ?  418.  Explain  the  operation  of  cupping-glasses.  419.  Why  doe* 


1 76  PNEUMATICS. 

it,  ascends  till  it  reaches  a  region  of  the  atmosphere  as  rare 
as  itself. 

This  is  the  reason  why  Smoke  rises.  So,  when  a  fire  is  kindled  in  a  grate, 
a  draft  is  produced  in  the  chimney.  The  air  near  the  fire  is  rarefied  and  as- 
cends. A  vacuum  is  thus  formed  for  the  instant ;  cold  air  rushes  in  to  fill  it ; 
this  in  turn  is  heated  and  rises,  and  thus  there  is  a  constant  passage  of  hot 
air  up  through  the  chimney. 

To  show  the  ascent  of  hot  air,  take  a  circular 
piece  of  paper,  as  represented  in  Fig.  184,  and, 
commencing  at  any  point  of  the  outer  edge,  as 
A,  cut  in  the  direction  of  the  dotted  line.  Sup- 
port it  from  beneath  at  B  on  a  piece  of  wire,  and 
it  will  hang  down,  resembling  in  shape  the 
threads  of  a  cork-screw.  If  the  paper  thus  sus- 
pended be  held  over  a  hot  stove,  it  will  be  carried 
rapidly  round  by  the  ascending  currents  of  heat- 
ed air. 

420.  BALLOONS. — By  observing  the  rise  of  smoke,  Ste- 
phen and  Joseph  Montgolfier  \mon-g ol-fe-a'^  paper-manu- 
facturers in  France,  were  led  in  1782  to  the  invention  of 
balloons.  The  following  year,  they  exhibited  their  invention 
to  the  public. 

An  immense  bag  of  linen  lined  with  paper  was  prepared,  and  brought  di- 
rectly over  a  fire  of  chopped  straw.  In  a  few  minutes,  the  balloon  was  filled 
with  rarefied  air  and  released  from  its  fastenings.  It  rose  about  a  mile,  re- 
mained suspended  ten  minutes,  and  reached  the  ground  a  mile  and  a  half 
from  the  place  of  its  ascent.  The  same  year,  two  persons  ascended  to  a 
height  of  3,000  feet  in  the  basket  of  a  smoke  balloon,  and  came  down  in  safety. 

On  the  1st  of  January,  1784,  a  successful  ascent  was 
made  in  a  balloon  inflated  with  hydrogen.  This  gas  is  now 
generally  used  for  the  purpose,  on  account  of  its  superior 
buoyancy.  Even  when  badly  prepared,  it  has  but  one-sixth 
of  the  weight  of  air,  and  is  three  times  as  light  as  Montgol- 
fier's  mixture  of  heated  air  and  smoke. 

421.  Balloons  have  not  as  yet  been  turned  to  any  practical  use,  from  the 
fact  that  they  are  completely  at  the  mercy  of  the  wind,  no  way  of  steering 
them  having  been  devised.  A  theory  has  lately  been  put  forth,  however, 
that  at  a  certain  height  of  the  atmosphere  currents  are  always  setting  from 

heated  air  rise  ?  Explain  how  the  kindling  of  a  fire  causes  a  draft  in  a  chimney.  How 
may  the  ascent  of  hot  air  be  shown  ?  420.  By  whom  and  when  were  balloons  invent- 
ed? Describe  the  Montgolfiers1  balloon,  and  its  ascent.  "When  was  the  first  success- 
ful ascent  made  in  a  balloon  inflated  with  hydrogen  ?  Why  is  hydrogen  now  used  for 


NAVIGATION   OF  THE  AIE.  177 

west  to  east ;  if  this  be  so,  aerial  voyages  may  be  made  with  tolerable  cer- 
tainty, at  least  in  one  direction.  The  theory  in  question  has  been  in  part 
confirmed  by  a  balloon  voyage  (the  most  remarkable  on  record)  made  July  1, 
1859.  Four  persons  started  from  St.  Louis,  and  in  19  hours,  40  minutes,  land- 
ed in  Jefferson  Co.,  N.  Y.,  near  Lake  Ontario, — having  travelled  about  1,000 
miles,  at  a  rate  exceeding  that  of  the  fastest  railroad  train. 

422.  Long  before  the  invention  of  balloons,  attempts  were  made  to  navi- 
gate the  air.  At  different  periods  not  long  after  the  Christian  era,  adventur- 
ous men  launched  themselves  from  the  tops  of  high  buildings,  and  with 
different  sorts  of  apparatus  which  they  had  prepared  moved  a  short  distance 
through  the  air.  Mechanical  contrivances  resembling  wings  were  more  than 
once  resorted  to ;  but  several  who  tried  them  met  with  serious  accidents, 
and  it  was  at  last  proved  that  wings  sufficiently  large  to  support  a  man  iu 
the  air  would  be  too  heavy  for  him  to  move. 

The  Air-pump. 

423.  The  Air-pump  is  an   instrument 
used  for  removing  the  air  from  a  vessel 
called  a  Receiver.     Receivers  are  made  of 
glass,  and  are  usually  of  the  shape  repre- 
sented in  Fig.  185. 

424.  INVENTION   OF  THE  AIR-PUMP. — 
The  air-pump  was  invented  1654  A.  D.,  by 

Otto  Guericke  [go, '-re-Ted^  burgomaster  of         A  BECEIVER. 
Magdeburg,  Germany. 

Guericke's  first  attempt  to  obtain  a  vacuum  was  made 
with  a  barrel  full  of  water.  Having  closed  it  tightly,  he 
applied  a  pump  to  the  lower  part  and  commenced  drawing 
off  the  water.  Could  he  have  done  this  and  kept  the  air 
out,  a  vacuum  would  have  been  formed ;  but  he  had  not 
proceeded  far,  when  the  air  from  without  began  to  force 
its  way  with  a  loud  noise  through  the  seams  of  the  barrel. 
To  remedy  the  difficulty,  Guericke  substituted  a  metallic 
globe  for  his  barrel  of  water,  and  the  experiment  was  then 
successful. 

inflating  balloons  ?  421.  "Why  have  not  balloons  been  turned  to  practical  use  ?  What 
remarkable  voyage  has  lately  been  made  ?  422.  Give  an  account  of  the  early  at- 
tempts to  navigate  the  air.  423.  What  is  the  Air-pump  ?  Of  what  are  receivers  made  ? 
424.  By  whom  and  when  was  the  air-pump  invented  ?  Give  an  account  of  Guericke's 
first  attempt  to  obtain  a  vacuum.  How  did  he  finally  succeed?  Describe  Gue- 

8* 


178 


PNEUMATICS. 


Great  improvements  have  been  made  on  the  rude  air-pump  employed  by 
Guericke ;  yet,  imperfect  as  his  instrument  was,  it  produced  results  of  deep 
interest  to  the  learned  men  of  that  day.     His  most  famous  experiment  was 
performed  before  the  Emperor  of  Germany  and  his  court.    Two  hollow  me- 
Fig.  186.  tallic  hemispheres  of  great  size  were  prepared,  fitting  each 

other  so  closely  as  to  form  an  air-tight  globe.  From  this 
globe  the  air  was  removed  with  the  pump,  and  a  stop-cock 
prevented  any  new  air  from  entering.  Fifteen  horses 
were  then  harnessed  to  each  hemisphere ;  but  their  united 
strength  was  unable  to  effect  a  separation,  so  tightly  were 
the  two  parts  held  together  by  atmospheric  pressure.  On 
turning  the  stop-cock  and  readmitting  the  air,  they  fell 
asunder  by  their  own  weight. 

425.  This  experiment  is  often  repeated  at  the  present 
day,  on  a  small  scale.    The  Magdeburg  hemispheres,  as  they 
are  called  from  Guericke's  native  city,  are  represented  in 
Fig.  186.    They  are  fixed  to  the  plate  of  an  air-pump,  in- 
stead of  a  receiver ;   and  on  exhausting  the  air  they  are 
MAGDEBURG       pressed  together  so  tightly  that  two  men  can  not  pull  them 
HEMISPHERES.       apart. 

426.   SINGLE-BARRELLED  AIR-PUMP. — A  single-barrelled 
Fig.  187.  air-pump    is    repre- 

sented in  Fig.  187. 
A  is  a  receiver  with 
its  edge  carefully 
ground,  resting  on  a 
plate  near  the  centre 
of  the  stand.  In 
this  plate  there  is  a 
hole  leading  into  a 
pipe  beneath,  which 
connects  the  receiv- 
er with  the  barrel  B. 
The  lower  part 
of  the  barrel  is  rep- 
resented as  cut  away 
in  the  figure,  in  or- 
der to  show  the  interior.  A  piston  is  tightly  fitted  to  it, 
containing  a  valve  opening  upward,  and  connected  with  a 

ricke's  famous  experiment  before  the  Emperor  of  GermaDy.    425.  Describe  the  ex- 
periment with  the  Magdeburg  hemispheres.    426.  Describe  the  single-barrelled  air- 


THE  SINGLE-BARRELLED  AIR-PUMP. 


THE  AIK-PUMP. 


179 


Fig.  188. 


handle,  by  which  it  may  be  worked  up  and  down.  At  the 
base  of  the  barrel  there  is  another  valve,  also  opening  up- 
ward. 

427.  Operation.— The  plate  having  been  carefully  dusted  and  rubbed  with 
a  little  oil,  the  receiver  is  placed  on  it,  and  the  piston  is  drawn  up.  A  vac- 
uum is  thus  formed  in  the  lower  part  of  the  cylinder,  and  the  air  in  the  re- 
ceiver, by  reason  of  its  elasticity,  pushes  up  the  lower  valve  and  enters  the 
barrel.  The  piston  is  now  in  turn  driven  down ;  the  pressure  at  once  closes 
the  lower  valve,  while  the  resistance  of  the  air  in  the  barrel  opens  the  valve 
to  the  piston.  Through  the  latter  the  air  passes  out,  and  by  the  time  the 
piston  has  reached  the  bottom,  it  has  all  escaped.  The  piston  is  then  again 
raised,  and  the  whole  operation  is  repeated, — a  barrel-full  of  air  being  drawn 
out  from  the  receiver  as  often  as  the  piston  ascends,  and  expelled  from  the 
barrel  as  it  descends.  At  last  the  air  in  the  receiver  becomes  so  rare  that  it 
has  not  sumcient  elasticity  to  open  the  valve  at  the  base  of  the  barrel.  After 
this  the  exhaustion  can  not  be  carried  any  further.  A  perfect  vacuum,  there- 
fore, is  not  produced ;  but  the  air  is  rarefied  to  such  a  degree  that  we  speak 
of  it  as  such. 

428.  DOUBLE-BAR- 
RELLED   AlR-PUMP. 

The  double-barrelled 
air-pump  (see  Fig. 
188)  acts  on  the  same 
principle  as  the  above, 
but  exhausts  the  air 
more  quickly  in  con- 
sequence of  having 
two  barrels  and  pis- 
tons. A  section  of 
the  instrument  is  rep- 
sented  in  Fig.  189,  from  which  its  working  will  be  readily 
understood. 

A  and  B  are  the  barrels,  in  which  the  pistons,  C,  D,  work 
up  and  down.  Each  piston  is  connected  with  a  rack,  E,  F, 
the  teeth  of  which  work  in  the  cog-wheel  G,  turned  by 
the  handle  M.  When  C  is  raised,  D  is  lowered;  and 
when  C  is  lowered,  D  is  raised.  H I  is  the  passage  which 

pump,  as  represented  in  Fig.  1ST.  Describe  the  interior  of  the  barrel.  42T.  How 
does  the  single-barrelled  air-pump  operate  ?  428.  How  does  the  double-barrelled  air- 
pump  differ  from  the  single-barrelled?  Describe  the  operation  of  the  double -bar- 


DOUBLE-BARRELLED   AIR-PUMP. 


180 


PNEUMATICS. 


- 189.  connects  the  bar- 

rels with  the  re- 
ceiver J.  K  is  a 
stop-cock  by  which 
the  connection  may 
be  cut  off.  L  is  a 
tube  resting  at  one 
end  in  a  small  ves- 
sel of  mercury,  and 
at  the  other  con- 
nected with  the  re- 
ceiver. This  tube 
is  called  a  barome- 
ter gauge.  As  the  air  in  the  receiver  is  rarefied,  the  external 
atmospheric  pressure  on  the  mercury  in  the  vessel  causes 
it  to  rise  in  the  tube ;  the  degree  of  rarefaction  is  there- 
fore shown  by  the  position  of  the  mercury. 

429.  EXPERIMENTS  WITH  THE  AIR-PUMP. — With  the  air- 
pump  and  different  pieces  of  apparatus  which  accompany 
it,  may  be  performed  a  variety  of  experiments,  illustrating 
the  properties  of  air. 

Fio>  19Q  430.  The  Hand-glass.— The  Hand-glass  (Fig.  190) 

is  a  receiver  open  at  both  ends.  Set  the  large  end 
on  the  plate  of  the  air-pump,  and  place  the  hand 
flat  upon  tne  top.  As  soon  as  the  pump  is  worked, 
the  pressure  of  the  atmosphere  is  felt.  When  the 
air  is  exhausted,  the  hand  can  hardly  be  removed 
from  the  glass ;  on  readmitting  the  air  through  a 
stop-cock,  it  is  raised  without  difficulty.  The  ex- 
pansion of  the  air  in  the  palm  of  the  hand  is  shown 
by  the  redness  of  the  flesh,  and  its  puffing  out  while 
over  the  exhausted  glass. 
431.  The  Apple-cutter.— The  Apple-cutter  (Fig.  191)  is  a  metallic  cylinder 
with  a  sharp  upper  edge.  An  apple  that  fits  it  closely  having  been  placed 
on  its  top,  the  air  is  exhausted.  The  pressure  of  the  atmosphere  forces  the 
apple  down  on  the  sharp  edge ;  the  middle  part  is  cut  out  and  falls  inside  of 
the  vessel. 


THE  HAND-GLASS. 


relied  air-pump,  with  the  aid  of  Fig.  189.  What  is  the  use  of  the  barometer  gauge  ? 
430.  What  is  the  Hand-glass  ?  Describe  the  experiment  with  the  hand-glass.  Wha; 
sauses  the  redness  of  the  hand  ?  431.  What  is  the  Apple-cutter  ?  Describe  the  ex 


EXPERIMENTS   WITH   THE   AIR-PUMP. 


181 


THE    APPLE- 
CUTTEB. 

433. 


TILE    BLAUUKK- 
GLASS. 


Fig.  193. 


Fig.  191.  432.  The,  Bladder-glass.— Over  the  large         Fig.  192. 

end  of  the  hand-glass  tie  a  wet  bladder,  as 
shown  in  Fig.  192.  When  the  bladder  has 
become  dry,  place  the  open  end  on  the  plate, 
and  exhaust  the  air  from  the  glass.  The 
pressure  of  the  atmosphere,  unsupported 
from  within,  soon  bursts  the  bladder  with  a 
loud  noise.  If  a  piece  of  thin  india  rubber 
be  substituted  for  the  bladder,  it  will  be 
drawn  in  and  distended,  till  it  covers  near- 
ly the  whole  inside  of  the  glass. 

lass.— The  Lungs-glass  (Fig.  193)  illus- 
trates the  elasticity  of  air.  It  is  a  small  glass  globe  with 
a  metallic  stopper.  Through  this  stopper  passes  a  tube, 
to  the  lower  part  of  which  a  bladder  is  tied.  The  whole  is 
placed  under  a  receiver,  and  the  air  exhausted.  The  air 
in  the  bladder,  communicating  through  the  tube  with  the 
receiver,  is  gradually  rarefied.  The  air  around  it  in  the 
Fig.  194.  glass>  having  no  communication 

with  the  receiver,  remains  of  the 
same  density.  Owing  to  its  pres- 
sure, the  bladder  becomes  shrivelled 
when  the  receiver  is  exhausted; 
but,  on  the  readmission  of  the  air,  it  resumes  its  former 
dimensions.  This  movement,  regularly  repeated,  re- 
sembles the  action  of  the  lungs  in  breathing,  and  hence 
the  name  given  to  the  apparatus. 

434.  Vacuum  Fountain. — Fig.  194  represents  9 
tall  glass  receiver,  terminating  at  the  bottom  in  a  me/ 
tallic  cap,  through  which  a  tube  passes.  This  tube  is 
furnished  with  a  stop-cock,  and  a  screw,  by  means  of 
which  it  may  be  fastened  to  the  plate  of  an  air-pump. 
A  jet  communicating  with  the  tube  rises  into  the  re- 
ceiver. Screw  this  apparatus  to  the  plate  of  the  pump, 
exhaust  the  air,  and  close  the  stop-cock.  Then  un- 
screw the  whole,  place  the  lower  end  of  the  tube  in  a 
vessel  of  water,  and  open  the  stop-cock.  The  pres- 
sure of  the  atmosphere  will  force  the  water  up  through 
the  tube  and  jet  into  the  vacuum,  forming  a  beautiful 
miniature  fountain. 

Another  mode  of  producing  a  vacuum  fountain  is 
VACUUM  FOUNTAIN.       with  the  apparatus  shown  in  Fig.  195.     It  consists  of 


THE  LUNGS-GLASS. 


periment  with  the  apple-cutter.  432.  How  is  the  experiment  with  the  bladder-glass 
performed  ?  433.  "What  does  the  Lungs-glass  illustrate  ?  "What  does  it  consist  of?  De- 
scribe the  experiment.  "Why  is  the  lungs-glass  so  called  ?  434.  "What  does  Fig.  194  rep- 
resent ?  How  is  the  vacuum  fountain  produced  ?  Describe  another  mode  of  producing 


182 


PNEUMATICS. 


Fig.  195.  a  glass  vessel  with  an  air-tight  stopper,  through  which  a 
tube  extends  almost  to  the  bottom.  The  vessel,  nearly  filled 
with  water,  is  placed  under  a  tall  receiver,  and  the  air  ex- 
hausted. The  elasticity  of  the  air  within  the  vessel,  not  be- 
ing counterbalanced  by  any  pressure  from  without,  forces  the 
water  through  the  tube  in  the  form  of  a  fountain. 

435.  Bottle  Imps.— The  bottle  imps,  described  in  §  397, 
may  be  made  to  dance  up  and  down  in  a  jar  of  water  in  an 
exhausted  receiver.    These  figures  are  hollow  and  contain 
air.    When  the  receiver  is  exhausted,  the  pressure  on  the  surface  of  the 
water  being  removed,  the  air  in  the  figures  expands  and  drives  out  some  of 
the  water.      This  diminishes   their  specific  gravity,  and  causes  them  to 


Fig.  196. 


rise.  When  the  air  is  readmitted,  the  pressure  is  restored, 
the  air  in  the  figures  is  compressed,  water  enters,  their 
specific  gravity  is  increased,  and  they  sink. 

436.  The  Mercury  Shower. — On  an  open-mouthed  re- 
ceiver, D,  place  the  cup  A,  in  the  bottom  of  which  is  a  plug 
of  oak  wood,  B,  projecting  downward  about  two  inches. 
Put  some  mercury  in  A,  and  set  the  saucer  C  beneath  the 
oaken  plug.    Exhaust  the  air  from  D,  and  the  mercury 
will  soon  be  forced  by  atmospheric  pressure  through  the 
pores  of  the  oak,  and  fall  into  the  saucer  in  a  silvery 
shower. 

437.  The  Weight-lifter. — This  is  an  apparatus  with 
which  the  pressure  of  the  atmosphere  is  made  to  lift  a 

heavy  weight  (see  Fig.  197).  A  is  a  cylinder  attached  to  a  frame,  firmly  sup- 
ported by  three  legs.  On  the  bottom  of  the  cylinder  rests  a  closely  fitting 
piston,  to  which  the  platform  F  is  attached.  A  tube,  B  C,  connects  the  in- 
terior of  the  cylinder  with  the  plate  E  of  the  pump  D.  When  the  air  is  ex- 
hausted from  A,  the  pressure  of  the  atmosphere  raises  the  piston,  together 
with  the  platform  and  its  contents,  the  whole  length  of  the  cylinder.  Atmos- 
pheric pressure  being  15  pounds  to  the  square  inch,  the  number  of  pounds 
that  can  be  lifted  by  a  given  cylinder  may  be  found  by  multiplying  its  area 
expressed  in  inches  by  15. 

438.  It  has  been  proposed  to  transmit  mails  between  distant  points,  by 
atmospheric  pressure,  on  the  principle  of  the  weight-lifter.  A  strong  me- 
tallic tube,  perfectly  smooth  on  the  inside,  would  have  to  be  laid  between 
the  places,  and  a  piston  tightly  fitted  to  it.  Large  air-pumps,  worked  by\ 
steam,  would  be  placed  at  both  ends  of  the  tube.  The  mail  being  attached 
to  the  piston  at  one  end  of  the  line,  the  pumps  at  the  other  would  be  set  in 
motion.  A  partial  vacuum  would  be  produced,  and  atmospheric  pressure 
would  drive  the  piston  through  the  tube  at  a  rate  estimated  at  500  miles  an 

a  vacuum  fountain.  435.  How  may  bottle  imps  be  made  to  dance  up  and  down  in  a 
jar  of  water?  Explain  the  principle.  436.  How  is  the  mercury  shower  produced? 
437.  "What  is  the  Weight-lifter  ?  Describe  it,  and  its  mode  of  operating.  How  many 
pounds  will  a  given  cylinder  lift  ?  438.  To  what  has  it  been  proposed  to  apply  this 


EXPERIMENTS   WITH  THE  AIK-PUMP. 

Fig.  197. 


183 


hour.    Such  is  the  theory ;  whether  it  can  be  practically  applied,  remains  t« 
be  proved. 

439.  Vacuum  Sell. — This  apparatus  is  intended 
to  show  that  air  is  essential  to  the  production  of 
sound.    A  bell  is  so  fixed  under  a  receiver  that  it 
can  be  rung  by  pushing  down  a  sliding-rod  which 
passes  through  the  top.    When  rung  before  the  re- 
ceiver is  exhausted,  the  bell  is  distinctly  heard; 
but,  when  the  air  is  withdrawn,  it  is  almost  inaudi- 
ble.  If  a  perfect  vacuum  could  be  produced,  it  would 
not  be  heard  at  all. 

440.  Freezing  Apparatus. — Water  may  be  frozen 
in  a  vacuum,  with  the  apparatus  shown  in  Fig.  199. 

Having  placed  the  liquid 


Fig.  199. 


in  a  shallow  vessel  over  a 
basin  containing  strong 
sulphuric  acid,  set  the 
whole  under  a  receiver 
and  exhaust  the  air.  Un- 
der the  diminished  pres- 


principle  ?    Give  the  theory  of  the  process.    439.  "What  is  the  apparatus  known  as  th« 
vacuum  bell  intended  to  show  ?    Describe  the  experiment    440.  Describe  the  freez- 


184  PNEUMATICS. 

sure,  the  water  is  rapidly  converted  into  vapor,  which  is  as  rapidly  absorbed 
by  the  acid.  The  continued  evaporation  cools  the  water  to  such  a  degree 
that  it  is  finally  covered  with  ice. 

441.  Miscellaneous  Experiments. — In  a  vacuum,  boiling  commences  at  a 
much  lower  temperature  than  in  the  air.     This  is  shown  by  placing  some 
hot  water  under  a  receiver  and  exhausting  tlie  air.     The  pressure  of  the  at- 
mosphere being  removed  from  its  surface,  the  water  soon  boils ;  but  it  conies 
to  rest  the  moment  that  air  is  readmitted.     For  the  same  reason,  water  boils 
at  a  lower  temperature  on  the  top  of  a  mountain  than  at  its  base,  as  has  often 
been  observed  by  travellers. 

442.  If  beer  is  placed  under  a  receiver  and  the  air  exhausted,  it  begins  to 
foam.    This  is  owing  to  the  elasticity  of  the  carbonic  acid  in  the  liquid,  rush- 
ing out  to  fill  the  vacuum.    If  the  air  is  readmitted,  the  beer  resumes  its 
usual  appearance. 

443.  A  shrivelled  apple  in  an  exhausted  receiver  is  puffed  out  to  its  full 
size  by  the  expansion  of  the  air  within. 

444.  If  a  vessel  of  water  containing  a  piece  of  wood,  a  vegetable,  oral- 
most  any  solid  substance,  is  placed  under  a  receiver,  and  the  air  is  exhaust- 
ed, minute  globules  of  air  can  be  seen  forming  on  the  surface  of  the  solid, 
and  sometimes  even  bubbling  up  through  the  water.     This  proves  the  poros- 
ity of  solids  and  the  presence  of  air  in  their  pores. 

445.  A  lighted  candle  in  an  exhausted  receiver  is  extinguished,  and  the 
smoke  falls  because  it  is  heavier  than  the  rarefied  air.     If  a  mouse,  rabbit,  or 
other  living  creature,  is  placed  under  a  receiver  and  the  air  is  drawn  off,  it 
immediately  shows  signs  of  distress,  and  soon  dies. 

446.  These  experiments  show  that  air  is  everywhere 
present,  and  is  essential  to  life  and  combustion.     In  a  vac- 
uum, animals  die,  vegetation  ceases,  and  sound  can  not  be 
produced. 

The  Condenser. 

447.  The  Condenser  (Fig.  200)  is  an  instrument  used 
for  forcing  a  large  quantity  of  air  into  a  given  vessel. 

Like  the  single-barrelled  air-pump,  the  condenser  con- 
sists of  a  cylinder,  A,  with  a  valve  at  its  base,  V,  and  a  pis- 
ton, P,  which  also  contains  a  valve,  tightly  fitted  to  it. 


Ing  apparatus,  and  the  experiment  with  it.  441.  At  what  temperature  does  boiling 
commence  in  a  vacuum,  compared  with  that  at  which  it  commences  in  the  air? 
How  is  this  shown?  What  is  said  of  the  hoiling  of  water  on  the  top  of  a  mountain  ? 
442.  What  phenomenon  is  presented  when  beer  is  placed  under  a  receiver  and  the  air 
exhausted?  443.  When  a  shrivelled  apple  is  so  placed?  444  How  is  the  presence 
of  air  in  the  pores  of  solids  proved  with  the  air-pump?  445.  How  is  it  shown  with 
the  air-pump  that  air  is  necessary  to  combustion  and  animal  life  ?  447.  What  is  th« 


THE   CONDENSER. 


185 


Instead  of  opening  upward,  however,  as  in  the 
air-pump,  these  valves  open  downward. 

448.  Operation. — The  condenser  having  been 
screwed  to  any  strong  vessel  in  which  it  is  desired 
to  condense  air,  the  handle  is  worked  up  and 
down.  A  vacuum  being  produced  below  the  pis- 
ton, as  it  ascends,  its  valve  is  opened  and  air 
rushes  in ;  while  the  valve  in  the  cylinder  is  closed 
by  the  pressure  of  the  air  in  the  vessel.  When 
the  piston  descends,  its  valve  is  closed  by  the 
pressure  of  the  air  in  the  cylinder,  while  the  other 
valve  opens  and  allows  this  air  to  be  driven  into  the  vessel. 
With  every  ascent  of  the  piston,  therefore,  the  cylinder  is 
filled  with  air,  and  with  every  descent 
this  cylinder-full  of  air  is  forced  into 
the  vessel. 

Air  is  condensed  in  the  chamber  of 
the  air-gun  (described  in  §  399)  by  the 
use  of  this  instrument. 

449.  Experiment. — An  interesting  experiment 
may  be  performed  with  the  condenser  and  the  ap- 
paratus represented  in  Fig.  201.  A  is  a  globe  half 
full  of  water,  with  a  tube,  B,  reaching  nearly  to  the 
bottom,  and  extending  upward  through  an  air-tight 
cap  till  it  terminates  in  a  screw  just  above  the  stop- 
cock D.  The  condenser,  having  been  screwed  on, 
is  worked  till  a  large  quantity  of  air  is  forced  into 
A.  The  stop-cock  is  then  closed,  the  condenser  is 
unscrewed,  and  a  jet-pipe,  C,  is  put  on  in  its  place. 
The  stop-cock  is  now  opened,  when  the  pressure  of 
the  condensed  air,  being  greater  than  that  of  the 
atmosphere,  forces  the  water  in  A  up  through  the 
jet,  making  a  beautiful  fountain. — This  experiment 
shows  that  the  elasticity  of  air  is  increased  by 
condensing  it. 

Pneumatic  and  Hydraulic  Machines. 

450.   THE   SIPHON. — The  Siphon,  represented  in  Fig. 


Condenser  ?    Describe  it.    448.  How  does  the  condenser  operate  ?    449.  Describe  an 
experiment  with  the  condenser  and  the  apparatus  represented  in  Fig.  201.  450.  "What 


186 


PNEUMATICS. 


TI1E  6IPHOX. 


202,  is  a  simple  instrument  for  drawing  off  liquids  from  a 
higher  to  a  lower  level.  It  is  nothing  more  than  a  bent 
tube,  with  one  leg  longer  than  the  other. 

Fig  202  ^°  use  ^e  siphon,  fill  it  with  some  liquid  and  then  invert 

it,  stopping  the  long  end  with  the  finger,  and  setting  the  short 
one  in  the  liquid  to  be  drawn  off.  Remove  the  finger,  and  the 
liquid  will  commence  flowing  from  the  long  end.  The  upward 
pressure  of  the  atmosphere  is  counterbalanced  bj  its  down- 
ward pressure  on  the  surface  of  the  liquid  to  be  drawn  off,  and 
the  liquid  in  the  tube  will  therefore  flow  in  the  direction  of  its 
greatest  weight.  As  it  flows,  a  vacuum  is  formed  in  the  tube, 
and  fresh  liquid  is  constantly  forced  up  into  the  short  leg. 
The  flow  continues  till  the  liquid  falls  below  the  extremity  of 
the  short  leg. 

451.  Some  siphons,  like  that  in  the  figure,  have  an  addi- 
tional tube,  open  at  the  upper  end  and  at  the  lower  communi- 
cating with  the  long  leg.  This  saves  the  trouble  of  turning 
the  siphon,  every  time  it  is  used,  to  fill  it  with  liquid ;  for, 
the  long  leg  being  stopped  with  the  finger  and  the  mouth  ap- 
plied to  this  additional  tube,  the  liquid  may  by  suction  readily  be  made  to 
fill  both  legs. 

452.  TANTALUS'S  CUP. — Fig.  203  represents  Tantalus's 
Cup,  which  is  simply  a  goblet  containing  a  siphon,  the  short 
Fig.  203.  leg  of  which  reaches  nearly  to  the  bottom, 
while  its  long  leg  passes  through  the  bottom 
and  extends  below.  The  siphon  is  concealed 
by  a  figure,  which  seems  to  be  trying  to 
drink.  Water  is  poured  in ;  but,  the  mo- 
ment it  reaches  the  lips  of  the  figure,  it  re- 
cedes, because  just  then  it  passes  the  turn 
of  the  siphon  and  begins  to  be  discharged 
below. 

TANTALUS'S  CUP.  453.  THE  LIFTING-PUMP. — The  Lift  in  g- 
pump  was  invented  by  Ctesibius  [te-sib'-e-us],who  flourished 
at  Alexandria,  in  Egypt,  250  B.  c.  Though  the  son  of  a 
barber  and  brought  up  to  his  father's  calling,  he  attained 
distinction  by  his  mechanical  abilities.  Several  ingenious 

is  the  Siphon  ?  How  is  it  used  ?  Explain  the  principle  on  which  it  works.  451.  What 
improvement  is  attached  to  some  siphons  ?  452.  Describe  Tantalus's  Cup,  and  the 
principle  on  which  it  works.  453.  Who  invented  the  Lifting-pump  ?  What  is  said 
of  Ctesibius?  454.  Of  what  does  the  lifting-pump  consist?  455.  Describe  its  mode 


THE  LIFTING-PUMP. 


187 


contrivances  for  raising  water  are  attributed  to  this  philos- 
opher, besides  the  clepsydra  already  described. 

454.  The  common  Lifting-pump  is  rep-  rig.  204 
resented  in  Fig.  204.  It  consists  of  a  cyl- 
inder, B  C,  to  which  is  fitted  the  air-tight 
piston  G,  containing  a  valve  opening  up- 
ward. A  is  called  the  suction-pipe ;  it 
must  be  long  enough  to  reach  the  water 
that  is  to  be  raised.  In  the  top  of  the 
guction-pipe  is  the  valve  H,  opening  upward 
into  the  cylinder.  E  is  a  handle,  by  which 
the  piston  may  be  worked.  F  is  a  spout, 
from  which  the  water  is  discharged. 

455.  Operation. — To  work  the  pump,  raise  the  pis- 
ton. As  it  ascends,  it  leaves  a  vacuum  behind  it,  and 
the  water  under  the  pressure  of  the  atmosphere  rushes 
up  through  A,  opens  H,  and  fills  the  cylinder  B  C.  The 
piston,  having  reached  the  top,  is  now  forced  back. 
Its  downward  pressure  at  once  closes  the  valve  H,  so 
that  the  water  can  not  return  into  the  suction-pipe ;  but 
the  valve  in  the  piston  opens,  and  through  it  the  water 
rushes  above  the  piston.  When  the  piston  has  reached 
the  bottom  of  the  cylinder,  it  is  again  raised ;  its  valve 
being  now  closed  by  the  downward  pressure,  the  water 
is  lifted  by  the  piston  into  the  reservoir  D,  whence  it  is 
discharged  by  the  spout.  Meanwhile,  the  second  time 
the  piston  rises,  a  vacuum  is  formed  below  it  as  before, 
and  the  whole  operation  is  repeated. 

456.  Thus  we  see  that  water  is  raised  in  pumps  by  at- 
mospheric pressure.     The  air  will  support  a  column  of  wa- 
ter from  32  to  34  feet  high.     To  this  elevation,  therefore, 
water  can  be  raised  with  the  lifting-pump ;  for  greater  dis- 
tances, the  forcing-pump  must  be  used. 

457.  THE  FORCING-PUMP. — The  Forcing-pump,  after  rais- 
ing a  liquid  through  its  suction-pipe,  does  not  discharge  it 
from  a  spout  above,  but  by  the  pressure  of  the  returning 
piston  drives  it  through  an  opening  in  the  side  below.    The 


THE  LIFTING-PUMP. 


of  operation.    456.  By  what  agency  is  water  raised  in  pumps  ?    How  high  a  column 
Will  atmospheric  pressure  support  ?    To  raise  water  to  a  greater  height,  what  must 


188 


PNEUMATICS. 


liquid  is  thus  forced,  either  directly  or  by  means  of  the 
pressure  of  condensed  air,  to  a  greater  height  than  it  could 
otherwise  attain. 

458.  Fig.  205  represents  one  form  of  Fig.  205. 
the  forcing-pump.     It  has  a  cylinder,  pis- 

ton,and  suction-pipe,  like  the  lifting-pump 
just  described;  but  there  is  no  valve  in 
the  piston.  Near  the  bottom  of  the  cylin- 
der enters  the  pipe  M,  which  communi- 
cates with  the  air-chamber  K,  by  the  valve 
P,  opening  upward.  The  tube  I,  open  at 
the  bottom  and  terminating  at  the  upper 
end  in  a  jet,  passes  through  the  air-tight 
top  of  the  chamber  K,  and  extends  nearly 
to  its  bottom. 

459.  Operation. — To  work  the  forcing- 
pump,   raise  the  piston.    A  vacuum  is 
formed ;   and  water,  from  the  reservoir 
below,  rushes  through  the  suction-pipe, 
opens  H,  and  fills  the  cylinder.     The  pis- 
ton is  now  pushed  back,  when  H  at  once 
closes.  The  water  in  the  cylinder  is  forced 
into  M,  raises  P,  and  enters  the  chamber 
K.    The  water  in  K  soon  rises  above  the 
mouth  of  the  tube  I,  and  begins  to  con- 
dense the  air  in  the  upper  part  of  the  chamber.    The  higher 
the  water  rises  in  K,  the  more  the  air  is  condensed,  and  its 
elasticity  increases  in  proportion.     Its  pressure,   therefore, 
soon  becomes  greater  than  that  of  the  atmosphere,  and  drives 
out  the  liquid  through  the  jet. 

Some  forcing-pumps  have  no  air-chamber,  but  drive  out 
the  liquid  by  the  direct  pressure  of  the  descending  piston.  In 
that  case,  the  discharge  is  by  successive  impulses ;  but,  when 
made  from  an  air-chamber,  it  is  continuous. 

460.  THE  FIRE-ENGINE. — The  Fire-engine  is  a  combina- 
tion of  two  forcing  pumps,  with  a  common  air-chamber 
and  suction-pipe.  Its  operation  will  be  understood  from 
Fig.  206. 

The  pistons,  C,  D,  are  attached  to  a  working-beam,  A  B,  turning  on  the 


THE  FORCING' 
PUMP. 


6e  used  ?  457.  What  is  the  principle  on  which  the  Forcing-pump  acts  ?  458.  De- 
scribe the  form  of  forcing-pump  represented  in  Fig.  205.  459.  Explain  its  operation. 
When  there  is  no  air-chamber,  how  does  the  forcing-pump  drive  out  the  liquid  ? 
460.  Of  what  does  the  Fire-engine  consist?  Describe  its  operation  with  Fig.  206. 


THE  FIRE-ENGINE. 


189 


pivot  K,  so  that  one  rises  as  the  other     ^^^  Fig.  206. 

descends.  They  are  driven  up  and 
down  by  brakes  attached  to  the  beam 
and  worked  by  a  number  of  men  on 
each  side.  F  is  the  suction-pipe.  H 
is  the  air-chamber,  and  E  a  pipe  ris- 
ing from  it,  to  which  a  flexible  leather 
hose  is  attached,  so  that  the  stream 
can  be  turned  in  any  direction.  The 
piston  D  in  Fig.  206  is  ascending,  fol- 
lowed by  a  stream  of  water  from  the 
reservoir  below,  the  valve  I  leading 
into  the  air-chamber  being  closed. 
The  piston  C,  on  the  other  hand,  is 
descending ;  its  lower  valve  is  closed, 
and  the  water  drawn  into  the  cylinder 
during  its  previous  ascent,  is  now  being  forced  into  H,  through  the  open 
valve  J. 

461.  The  fire-engine  is  one  of  the  most  powerful  forms  of  the  forcing- 
pump,  since  water  is  being  constantly  forced  into  the  air-chamber  by  one  of 
the  pistons,  and  the  air  is  violently  compressed.  With  a  good  engine,  a 
stream  can  be  thrown  more  than  100  feet  high. 

462.  THE  CENTRIFUGAL 
PUMP.— The  Centrifugal  Pump 
(Fig.  207)  is  an  instrument 
for  raising  water  by  the  com- 
bined effect  of  the  centrifugal 
force  and  atmospheric  pres- 
sure. 

It  consists  of  a  vertical 
axle,  AB,  and  one  or  more 
tubes,  C,  C,  fastened  to  it, 
extending  into  a  reservoir  of 
water  below,  and  branching 
off  towards  the  top  so  as  to 
bring  their  mouths  over  the 
circular  trough  D.  E  is  a 
spout  for  discharging  the  wa- 


TIIE   CENTRIFUGAL  PUMP. 


461.  "What  is  said  of  the  power  of  the  fire-engine  ?    How  high  can  a  stream  be  thrown 
with  a  good  engine  ?    4C2.  What  forces  are  brought  to  bear  in  the  Centrifugal  Pumpf 


190  PNEUMATICS. 

ter  from  the  trough.     Near  the  top  and  bottom  of  each 
tube  is  a  valve  opening  upward. 

463.  Operation. — When  the  pump  is  to  be  worked,  the  tubes  are  filled 
with  water,  which  is  prevented  from  escaping  by  the  lower  valves.  A  rotary 
motion  is  then  communicated  to  the  tubes  by  means  of  a  handle  attached  to 
the  axle.  The  centrifugal  force  at  once  acts  on  the  water  within,  causing  it 
to  open  the  valves  and  rush  forth  from  the  mouths  of  the  tubes.  As  it  as- 
cends, a  vacuum  is  left  behind  it,  into  which  water  is  driven  by  atmospheric 
pressure  from  the  reservoir  below.  Streams  are  thus  kept  pouring  into  the 
trough  as  long  as  the  rotary  motion  is  continued. 

A  large  centrifugal  pump,  worked  by  steam,  has  raised  no  less  than  1,800 
gallons  a  minute  to  a  considerable  height. 

464.  THE  STOMACH  PUMP. — The  Stomach  Pump  is  an 
instrument  for  injecting  a  liquid  into  the  stomach  of  a  poi- 
soned person  and  withdrawing  it,  without  removing  the 
apparatus.  The  stomach  is  thus  rinsed  out,  and  life  is  often 
saved. 

Fig.  208. 

r/^  *       _^ 

A 


THE   STOMACH   PUMP. 


Fig.  208  represents  the  stomach  pump.  A  syringe,  A, 
is  screwed  into  a  cylindrical  box,  B,  where  it  communicates 
with  a  short  metallic  tube.  This  tube  leads  on  either  side 
into  a  bulb,  which  is  connected  with  a  tube  of  india  rubber. 
Each  bulb  contains  a  movable  circular  valve  of  metal,  which 
fits  either  extremity,  and  may  be  made  to  close  either  by 
raising  the  opposite  side  of  the  instrument. 

Operation.  —  To  work  the  pump,  turn  the  syringe  so  as  to  depress  C  and 
elevate  D  ;  and  then  introduce  the  tube  F  into  the  patient's  stomach,  and  E 
into  a  basin  of  warm  water.  The  metallic  valves  fall  to  the  lowest  part  of 

Of  what  does  the'  centrifugal  pump  consist  ?  463.  What  is  its  mode  of  operation  ? 
What  has  been  effected  with  a  large  centrifugal  pump  worked  by  steam  ?  464.  For 
what  is  the  Stomach  Pump  used  ?  Describe  its  parts.  How  is  it  worked  ? 


EXAMPLES  FOB  PRACTICE.  191 

their  respective  bulbs,  which  brings  them  directly  opposite  where  they  are  in 
the  Figure.  Now  draw  out  the  handle  of  the  syringe.  A  vacuum  is  pro- 
duced ;  and  the  warm  water,  under  atmospheric  pressure,  rushes  up  to  fill 
it,  all  communication  with  F  being  cut  off  by  the  valve.  The  syringe  being 
thus  charged,  the  handle  is  pressed  back,  and  the  water,  prevented  from  re- 
turning into  E  by  the  valve,  is  forced  through  F  into  the  stomach.  Without 
removing  the  india  rubber  tube  from  the  stomach,  now  turn  the  instrument, 
so  as  to  raise  the  side  C  and  depress  D,  as  shown  in  the  Figure.  The  metal- 
lic valves  are  thus  thrown  to  the  opposite  extremities  of  their  bulbs,  and  by 
working  the  syringe  with  them  in  this  position,  the  contents  of  the  stomach 
are  drawn  off  and  discharged  into  the  basin.  The  syringe  is  thus  always 
charged  through  the  depressed  tube  and  emptied  through  the  elevated  one. 

465.  The  consideration  of  the  steam-engine,  the  great- 
est of  pneumatic  machines,  is  deferred  till  we  shall  have 
treated  of  the  mode  of  generating  steam  by  heat,  a  subject 
which  belongs  to  Pyronomics. 

EXAMPLES  FOB  PRACTICE. 

1.  (See  §  398.)  Under  a  pressure  of  one  atmosphere,  a  body  of  oxygen  fills  24 

cubic  inches,  audits  specific  gravity  is  1.1 11.  What  space  will  it  occupy, 
and  what  will  be  its  specific  gravity,  under  a  pressure  of  three  atmos- 
pheres ? 

2.  Some  hydrogen,  by  a  pressure  of  20  pounds  to  the  square  inch,  is  forced 

into  a  space  of  one  cubic  foot.  How  great  a  pressure  will  compress  it 
into  half  a  cubic  foot,  and  how  will  its  density  then  compare  with  what 
it  was  before  ? 

3.  Into  what  space  must  we  compress  10  cubic  inches  of  air,  to  double  its 

elastic  force  ? 

4.  (See  §  401.)  What  is  the  weight  of  600  cubic  inches  of  air?    What  is  the 

weight  of  the  same  bulk  of  water? 

5.  A  vessel,  full  of  air,  weighs  1,061  grains ;  exhausted,  it  weighs  but  1,000 

grains.    How  many  cubic  inches  does  it  contain  ? 

6.  (See  §  414.)  What  is  the  downward  atmospheric  pressure  on  the  roof  of  a 

house  containing  115,200  square  inches  ?  What  is  the  upward  atmos- 
pheric pressure  on  the  same  roof? 

7.  What  amount  of  atmospheric  pressure  is  supported  by  a  boy  whose  body 

contains  1,000  square  inches  of  surface? 

8.  (See  §  408.)  When  the  mercury  in  the  barometer  stands  at  29  inches,  at 

what  height  will  a  column  of  water  be  supported  by  the  atmosphere  ? 
[Solution. — The  specific  gravity  of  water  is  1 ;  that  of  mercury,  13.568. 

A  column  of  water  will  be  supported  at  the  height  of  29  X  13.568  inches.] 
f .  When  the  atmosphere  supports  a  column  of  water  32  feet  high,  how  high 

a  column  of  mercury  will  it  support? 
1C.  (See  Fig.  183.)  How  far  above  the  earth's  surface  would  the  mercury 

stand  only  two  inches  high  in  the  barometer  ? 


192  PYBONOMICS. 


CHAPTER  XIII. 

PYRONOMICS. 

466.  PYKONOMICS  is  the  science  that  treats  of  heat. 

Nature  of  Heat. 

467.  Heat  is  the  sensation  experienced  on  approaching 
a  warm  body. 

The  invisible  agent  that  produces  this  sensation  is  also 
called  Heat.  Another  name  for  it  is  Ca-lor'-ic. 

468.  Cold  is  the  opposite  of  heat.     It  is  not  a  positive 
agent,  but  merely  implies  a  greater  or  less  deficiency  of 
heat.     There  is  heat  in  all  substances ;  but  in  those  which 
we  call  cold,  it  exists  in  an  inferior  degree. 

469.  There  are  two  kinds  of  heat ;  Free,  or  Sensible, 
and  Latent. 

Free  or  Sensibly  Heat  is  heat  that  can  be  felt.  Latent 
Heat  is  heat  that  can  not  be  felt.  The  heat  of  a  fire  is  Free, 
or  Sensible  ;  the  heat  in  ice  is  Latent. 

470.  The  Temperature  of  a  body  is  the  amount  of  sen- 
sible heat  that  it  contains. 

\Ye  can  not  always  judge  correctly  of  a  body's  temperature  by  the  sensa- 
tion it  produces  when  -we  touch  it.  In  the  same  room,  for  instance,  are  a 
bar  of  iron  and  a  piece  of  cloth ;  they  must  be  of  the  same  temperature,  but 
the  iron  is  cold  to  the  touch  while  the  cloth  is  not.  This  is  because  the  iron 
carries  off  the  heat  more  rapidly  from  the  part  that  touches  it.  So,  if  one 
hand  be  cold  and  the  other  warm,  a  substance  which  to  the  former  seems 
hot,  to  the  latter  may  appear  just  the  reverse.  Our  sensations,  therefore,  are 
not  proper  criterions  by  which  to  judge  of  a  body's  temperature. 

466.  What  is  Pyronomics  ?  467.  What  is  Heat  ?  What  other  signification  has  the 
term  heat  f  What  other  name  is  therefor  it  in  this  sense?  468.  What  is  Cold? 
469.  How  many  kinds  of  heat  are  there  ?  What  is  Free  or  Sensible  Heat  ?  What  is 
Latent  Heat?  Give  examples.  470.  What  is  the  Temperature  of  a  body  ?  Can  we 
judge  of  a  body's  temperature  by  the  sensation  it  produces  when  we  touch  it  ?  State 


NATURE   OP   HEAT.  193 

471.  What  heat  is,  we  do  not  know. 

Some  think  that  it  is  not  a  material  substance,  but  results  from  the  vibra- 
tions of  the  particles  of  bodies.  Others  believe  it  to  be  an  exceedingly  sub- 
tile substance,  whose  particles  repel  each  other,  and  thus  give  it  a  tendency 
to  diffuse  itself,  while  they  have  a  strong  affinity  for  other  matter.  This  sub- 
stance, they  think,  enters  into  every  body,  and  keeps  its  particles  from  com- 
ing into  absolute  contact.  As  long  as  it  remains  at  rest,  it  may  be  latent ; 
but,  when  a  colder  body  approaches,  there  is  a  tendency  to  equalize  the  tem- 
perature ;  a  series  of  vibrations  are  produced  in  the  subtile  atmosphere  around 
«ach  particle,  and  the  heat  which  was  before  latent  becomes  sensible. 

Heat  seems  to  be  closely  connected  with  light.  The  one  is  generally  ac- 
companied by  the  other ;  and  to  some  extent,  as  will  appear  hereafter,  they 
are  governed  by  the  same  laws. 

472.  Heat  has  no  weight. 

Balance  a  piece  of  red-hot  iron  with  weights  in  a  sensitive  pair  of  scales ; 
the  same  weights  will  exactly  balance  it  when  it  has  become  cold.  Heat, 
therefore,  must  be  imponderable ;  or  the  loss  of  so  much  of  it  would  occa- 
sion a  perceptible  difference  in  the  weight  of  the  iron.  So,  if  a  piece  of  ice 
is  balanced  and  then  allowed  to  melt,  the  water  formed  will  weigh  precisely 
the  same  as  the  ice.  « 

Source*  of  Heat. 

*  473.  The  principal  Sources  of  Heat  are  four  in  number: 
— the  Sun,  Chemical  Action,  Mechanical  Action,  and  Elec- 
tricity. 

474.  THE  SUN",  A  SOURCE  OP  HEAT. — The  Sun  is  the  great 
source  of  heat,  as  well  as  light,  to  the  earth. 

What  the  sun  is  composed  of,  that  it  has  thus  for  thousands  of  years 
poured  forth  undiminished  supplies  of  heat,  astronomers  can  not  determine. 
Some  think  that  the  whole  of  its  immense  mass  is  heated  to  such  a  degree  as 
to  make  it  luminous.  According  to  others,  the  great  body  of  the  sun  is  not 
luminous,  but  its  surface  is  covered  with  flames  from  which  rays  of  heat  and 
light  are  constantly  emitted.  In  either  case,  it  is  hard  to  explain  how  com- 
bustion can  be  continued  so  long  without  exhausting  the  material  by  which 
it  is  supported. 

475.  The  heat  at  the  sun's  surface  is  supposed  to  be 
more  intense  than  any  with  which  we  are  acquainted.     By 

some  facts  to  prove  this.  471.  What  is  heat  ?  What  do  some  think  it  results  from  ? 
What  do  others  bslieve  it  to  be  ?  How  do  the  latter  account  for  its  being  sometimes 
latent  and  sometimes  sensible?  "With  what  is  heat  connected?  472.  What  is  the 
•weight  of  heat?  Prove  this.  473.  What  are  the  principal  sources  of  heat?  474.  What 
Is  the  great  source  of  heat  to  the  earth  ?  What  two  theories  have  been  advanced  to 
account  for  the  sun's  heat  ?  475.  How  great  is  the  heat  at  the  sun's  surface  supposed 

9 


194  PYEONOMICS. 

the  time  it  reaches  us,  modified  by  the  immense  distance  it 
has  traversed,  it  is  just  sufficient  to  warm  the  earth  into 
fertility. 

The  sun  does  not  heat  all  parts  of  the  earth  alike.  This  is  because  its 
rays  strike  some  portions  perpendicularly  and  others  obliquely.  The  per- 
pendicular rays  are  absorbed  more  than  the  oblique  ones,  and  therefore  pro- 
duce a  greater  degree  of  heat  in  the  parts  on  which  they  strike.  For  the 
same  reason,  it  is  hotter  about  noon  than  any  other  time  of  day,  the  sun 
being  then  more  directly  over  head. 

The  variety  of  productions  in  different  parts  of  the  earth  is  owing  to  the 
difference  in  the  amount  of  heat  received  from  the  sun.  The  trees  and  plants 
of  the  tropics  are  quite  different  from  those  of  the  temperate  regions,  and 
these  again  are  unlike  those  of  cold  climates.  In  the  far  north  and  south,  so 
little  heat  is  received  that  vegetation  entirely  ceases. 

Fin.  209  476.  The  sun's  heat  may  be  increased 

by  collecting  a  number  of  its  rays  into 
one  point  called  a  Focus.  This  may  be 
done  with  a  convex  lens,  .or  glass  of  the 
shape  represented  in  Fig.  209.  With 
such  a  lens,  three  feet  in  diameter,  the 
metals  have  been  melted. 

A  similar  effect  may  be  produced  with  concave 
mirrors,  so  arranged  as  to  reflect  the  rays  that  strike 
them  to  one  and  the  same  focus.  When  the  Romans 
were  besieging  Syracuse,  213  B.  c.,  Archimedes  is 
said  to  have  used  a  number  of  metallic  mirrors  with 
such  effect  as  to  set  fire  to  their  fleet.  The  experi- 
ment has  been  repeated  in  modern  times.  Buffoii, 
with  a  combination  of  168  mirrors,  showed  that 

tarred  planks  could  be  set  on  fire  at  a  distance  of  150  feet,  and  that  at  60  feet 

silver  could  be  fused. 

477.  Heat  below  the  Earth? s  Surface. — The  sun's  heat, 
even  when  it  falls  perpendicularly  on  the  surface,  does  not 
penetrate  into  the  earth  farther  than  100  feet.  Beyond 
this  depth,  all  the  heat  that  is  felt,  comes,  not  from  the 
sun,  but  from  the  interior  of  the  earth. 

to  be  ?    Why  is  it  less  intense  when  it  reaches  us  ?    Why  does  not  the  sun  heat  all 
parts  of  the  earth  alike  ?  To  what  is  the  variety  of  productions  in  different  parts  of 
earth  owing  ?    476.  How  may  the  sun's  heat  be  increased  ?    In  what  other  way  may  i 
similar  effect  be  produced  ?    What  did  Archimedes  accomplish  with  a  number  of 
tallic  mirrors  ?    Give  an  account  of  Buffon's  experiment.     477.  What  is  the  great 


SOUKCES   OP   HEAT.  195 

As  we  descend  below  the  earth's  surface,  the  temperature  increases  about 
one  degree  for  every  45  feet.  At  this  rate,  water  would  boil  at  a  depth  of 
less  than  two  miles,  and  at  125  miles  all  known  substances  would  be  melted. 
It  is  thought,  therefore,  that  the  great  mass  of  the  interior  of  the  earth  is  in 
a  state  of  fusion.  The  discharge  of  melted  earthy  matter,  called  lava,  during 
the  eruption  of  volcanoes,  goes  to  prove  this ;  while  the  hot  springs  in  differ- 
ent parts  of  the  world  (particularly  numerous  in  Iceland)  show  that  a  high 
temperature  prevails  at  no  very  great  depth.  At  the  surface  this  internal 
heat  is  not  perceptible,  because  the  outer  crust  of  the  earth  is  a  bad  conductor. 

478.  CHEMICAL  ACTION,  A  SOURCE  OF  HEAT. — When,  by 
combining  two  or  more  substances,  we  produce  a  new  sub- 
stance totally  different  in  its  properties  from  either,  we  say- 
that  Chemical  Action  has  taken  place.     Such  action  is  al- 
ways accompanied  with  an  increase  of  temperature.      If, 
for  instance,  we  mix  equal  quantities  of  sulphuric  acid  and 
water,  chemical   action  takes  place,  a  new   substance   is 
formed,  and  heat  is  given  out.    The  heat  produced  by  chem- 
ical action  is  sometimes  sufficient  to  ignite  inflammable  sub- 
stances.   Thus  a  drop  of  sulphuric  acid  will  set  fire  to  a 
mixture  of  sugar  and  chlorate  of  potassa. 

479.  Combustion. — One  of  the  commonest  processes  in 
which  chemical  action  is  exhibited,  is  Combustion,  or  Burn- 
ing.    This  is  the  great  source  of  artificial  heat,  as  the  sun 
is  of  natural  heat. 

Combustion  is  nothing  more  than  a  chemical  union  of  the  oxygen  of  the 
air  with  the  combustible  body  or  some  of  its  elements.  Latent  heat  is  given 
out,  by  which  the  gases  or  vapors  produced  are  rendered  luminous ;  and 
hence  what  we  call  Flame.  The  rise  of  temperature  is  proportioned  to  the 
rapidity  with  which  the  chemical  union  takes  place ;  and  this  depends  in  a 
great  measure  on  the  amount  of  oxygen  supplied. 

If  we  wish  to  make  a  fire  hotter,  we  have  only  to  bring  more  air  in  con- 
tact with  the  fuel.  This  may  be  done  with  a  bellows,  or  in  the  case  of  grates 
with  a  blower.  To  fill  the  vacuum  produced  by  the  ascent  of  the  heated  air 
through  the  chimney,  cold  air  must  enter ;  by  putting  on  the  blower,  we  pro- 
distance  to  which  the  sun's  heat  penetrates?  Beyond  this  depth,  whence  is  the  heat 
derived  ?  Descending  below  the  earth's  surface,  at  what  rate  does  the  temperaturo 
increase  ?  At  what  depth  would  water  boil  ?  How  great  would  the  temperature  be 
•t  a  depth  of  125  miles  ?  In  what  state  is  the  interior  of  the  earth  supposed  to  be  ? 
"What  phenomena  support  this  opinion  ?  478.  When  does  Chemical  Action  take  place  ? 
With  what  is  chemical  action  always  accompanied  ?  Give  an  example.  479.  In  what 
common  process  is  chemical  action  exhibited  ?  Whut  is  Combustion  ?  What  is  the 
cause  of  flame  ?  To  what  is  the  rise  of  temperature  proportioned  ?  What  must  be 


196  PYEONOMICS. 

vent  it  from  entering  anywhere  except  at  the  bottom  of  the  grate,  and  cause 
what  does  enter  to  pass  through  the  ignited  coals,  thus  increasing  their  sup- 
ply of  oxygen. 

480.  Animal  Heat. — To  Chemical  Action  is  attributable 
Animal  or  Vital  Heat, — that  is,  the  heat  generated  in  all 
organic  beings  that  possess  life. 

Different  living  creatures  have  different  degrees  of  ani- 
hial  heat.  Birds  have  the  most ;  beasts  come  next ;  then 
fish  and  insects.  In  the  same  class  of  animals,  however,  the 
amount  of  vital  heat  is  nearly  uniform  ;  and  under  ordinary 
circumstances  it  remains  the  same,  whether  the  surround- 
ing medium  be  warm  or  cold.  Other  things  being  equal, 
the  heat  of  the  human  body  is  as  great  in  winter  as  in  sum- 
mer, in  the  frigid  as  in  the  torrid  zone.  We  do  not  feel 
equally  hot,  to  be  sure ;  but,  as  already  explained,  we  must 
not  judge  of  temperature  by  our  feelings. 

481.  Animal  heat  is  produced  by  a  process  similar  to  combustion.    When 
we  breathe,  air  is  taken  into  the  lungs,  where  it  comes  in  contact  with  par- 
ticles of  carbon  contained  in  the  blood.     This  carbon  unites  chemically  with 
the  oxygen  of  the  air  inhaled,  and,  as  in  the  case  of  combustion,  latent  heat 
is  evolved.    The  heat  is  less  than  that  produced  by  combustion,  because  the 
particles  of  carbon  are  extremely  small. 

As  in  combustion,  whatever  increases  the  supply  of  oxygen  increases  the 
animal  heat.  Running  or  bodily  exertion  of  any  kind,  makes  us  hotter,  be- 
cause it  quickens  the  circulation  of  the  blood,  obliges  us  to  breathe  faster, 
and  thus  brings  more  air  (and  consequently  more  oxygen)  into  the  lungs. 

482.  The  carbon  consumed  comes  from  the  food  we  eat.    Greasy  food  gen- 
erates it  most  plentifully.     In  winter,  therefore,  when  we  need  an  abundance 
of  carbon,  we  eat  meat  more  freely  than  in  summer,  when  we  seek  to  reduce 
our  vital  heat  as  much  as  possible.    So,  the  inhabitants-  of  cold  regions  con- 
sume more  greasy  food  than  those  of  warmer  climates.    The  Esquimaux 
thrive  on  fish-oil  and  seals'  fat,  which  to  the  people  of  th«  tropics  would  be 
neither  palatable  nor  wholesome. 

483.  MECHANICAL  ACTION,  A  SOURCE  OF  HEAT. — Mechan- 

done,  if  we  wish  to  make  a  fire  hotter?  480.  What  is  Animal  or  Vital  heat?  To 
what  is  it  attributable  ?  What  is  said  of  animal  heat  in  different  living  creatures? 
In  the  same  class  of  animals  ?  Does  it  differ  in  different  seasons  ?  481.  How  is  ani- 
mal h.eat  produced  ?  Why  is  it  less  than  the  heat  produced  by  combustion  ?  How  is 
animal  heat  increased?  Give  examples.  482.  How  is  the  carbon  consumed,  pro- 
duced ?  What  sort  of  food  generates  carbon  most  plentifully  ?  What  follows,  with  re- 
epect  to  our  diet  at  different  seasons  ?  How  does  the  diet  of  the  inhabitants  of  cold 
regions  compare  with  that  of  tropical  nations  ?  483.  What  is  the  third  source  of  heat  1 


SOUKCES   OP   HEAT.  197 

ical  Action  is  a  familiar  source  of  heat.  Under  this  head 
are  embraced  Friction  or  Rubbing,  and  Percussion  or  Strik- 
ing. By  compressing  the  particles  of  a  body,  mechanical 
action  forces  out  its  latent  heat  and  makes  it  sensible. 

484.  Heat  from  Friction. — Touch  a  row-lock,  in  which 
an  oar  has  been  rapidly  plying,  or  a  gimlet  that  has  just 
been  vigorously  worked,  and  you  will  feel  the  heat  pro- 
duced by  friction.     Rub  a  metallic  button  to  and  fro  on  a 
dry  board,  and  you  will  soon  make  it  so  hot  that  you  can 
not  bear  your  finger  on  it.     By  drawing  a  match  across  a 
rough  surface,  you  develop  heat  enough  to  ignite  it.     By 
rubbing  two  pieces  of  ice  together,  in  a  freezing  tempera- 
ture, latent  heat  is  liberated  in  sufficient  quantities  to  melt 
them. 

Machinery  has  been  ignited  by  the  rubbing  of  its  parts  on  each  other. 
Savages  kindle  a  fire  by  rubbing  two  dry  sticks  violently  together.  In  bor- 
ing a  brass  cannon,  immersed  in  water  by  way  of  experiment,  sufficient  heat 
has  been  generated  to  boil  the  water  in  two  hours  and  a  half.  The  fric- 
tion of  two  large  iron  plates  has  even  been  employed  as  a  practical  source 
of  heat. 

It  is  to  be  observed  that  in  all  the  above  cases  heat  is  produced  by  the 
friction  of  solids.  The  friction  of  fluids  is  insufficient  to  generate  heat. 

485.  Heat  from  Percussion. — By  striking  flint  and  steel 
together,  we  develop  sufficient  heat  to  ignite  the  minutft 
fragments  broken  off,  and  produce  sparks.    In  like  manner, 
the  hammer  of  a  gun,  descending  on  a  percussion-cap,  sets 
fire  to  the  fulminating  mixture  of  which  the  cap  is  made. 

A  nail  may  be  made  red-hot  by  hammering  it  rapidly  on  an  anvil.  Be- 
fore lucifer  matches  were  invented,  blacksmiths  used  to  ignite  sulphur 
matches  and  kindle  their  forge-fires  with  a  nail  hammered  to  a  red  heat. 

By  violent  and  quick  compression,  enough  heat  can  be  set  free  from  air 
to  ignite  tinder.  This  is  done  with  the  Fire  Syringe  (see  Fig.  210).  In  the 
extremity  of  the  piston  is  a  small  cavity,  in  which  some  tinder  is  placed. 
When  the  piston  is  driven  rapidly  down,  the  air  in  the  barrel  is  compressed, 

What  are  included  under  this  head  ?  How  is  it  that  mechanical  action  produces  heat? 
484.  State  some  familiar  cases  in  which  heat  is  produced  by  friction.  What  is  some- 
times the  effect  of  friction  on  machinery  ?  How  do  savages  kindle  their  fires  ?  How 
great  a  heat  has  been  produced  by  boring  a  brass  cannon  ?  How  has  friction  been 
turned  to  practical  use  ?  What  is  said  of  the  friction  of  fluids  ?  485.  Give  some 
familiar  examples  of  the  production  of  heat  by  percussion.  How  did  blacksmiths 
formerly  kindle  their  forge-fires  ?  Describe  the  Fire-syringe,  and  the  experiment 


198  PYRONOMICS. 

Fig.  210.        latent  heat  is  evolved,  and  on  withdrawing  the  piston  the  tin- 
der will  be  found  ignited. 

If  a  body  is  compressed  by  violent  percussion  more  than 
once,  the  heat  produced  is  less  each  time,  until  at  last  all  the 
latent  heat  is  forced  out,  and  it  may  be  struck  or  hammered 
without  any  material  increase  of  temperature.  Iron,  when 
thus  deprived  of  its  latent  heat,  becomes  stiff  and  brittle.  The 
metals  generally  lose  their  ductility,  and  can  not  be  drawn  out 
into  wire  till  their  latent  heat  is  restored  by  subjecting  them 
to  the  action  of  fire. 

486.  ELECTRICITY,  A  SOURCE  OP  HEAT. — The 
passage  of  electricity  is  sometimes  attended  with 
intense  heat.  Lightning,  for  instance,  sets  fire 
to  trees  and  houses,  and  melts  metallic  bodies 
that  it  strikes.  The  heat  produced  by  the  gal- 
vanic battery  ignites  or  fuses  every  known  sub- 
stance. 

SYK1NGE. 

Diffusion  of  Heat. 

487.  Heat  tends  to  diffuse  itself  equally  among  bodies 
of  different  temperature.     So  strong  is  this  tendency,  that, 
unless  fresh  supplies  are  received,  the  hottest  body  soon  be- 
comes cool,  in  consequence  of  parting  with  its  heat  to  sur* 
rounding  objects  cooler  than  itself. 

488.  Heat  is  diffused  in  three  ways : — 

1.  By  CONDUCTION,  when  it  passes  from  one  particle  of 

a  body  to  another  in  contact  with  it.  If  one  end 
of  a  poker  is  placed  in  a  fire,  the  other  becomes 
heated  by  Conduction. 

2.  By  CONVECTION,  when  it  is  conveyed  by  the  actual 

motion  of  some  of  the  particles  of  a  body.  When, 
a  pot  of  water  is  placed  over  a  fire-,  the  particles 
at  the  bottom  are  first  heated,  and  ascend,  carry- 
ing heat  with  them  and  diffusing  it  by  Convec- 
tion. 


performed  with  it.  What  is  found,  when  a  body  is  violently  struck  more  than  once? 
What  change  is  produced  in  iron  thus  treated  ?  In  the  metals  generally  ?  4S6.  What 
is  the  fourth  source  of  heat?  Give  examples.  487.  What  is  the  tendency  of  heat? 
188.  In  how  many  ways  is  heat  diffused  ?  Name,  describe,  and  give  an  example  of 


DIFFUSION   OF   HEAT.  199 

3.  By  RADIATION,  when  it  passes  from  one  body  to  an- 
other not  in  contact  with  it,  leaping  over  the  in- 
tervening space.  A  joint  of  meat,  placed  before 
the  fire,  is  roasted  by  Radiated  Heat. 

489.  CONDUCTION. — Some  substances  allow  heat  to  pass 
freely  through  their  particles ;  others  do  not.     The  former 
are  called  Conductors  of  heat ;  the  latter,  Bad  Conductors, 
or  Non-conductors. 

As  a  general  rule,  dense  solids  are  conductors  of  heat ; 
porous  and  fibrous  solids,  as  well  as  liquids,  gases,  and  va- 
pors, are  bad  conductors* 

490.  The  Conductometer. — The  metals          rig.  211. 
are  all  good  conductors  of  heat,  but  some 

are  better  than  others.  This  is  shown  by 
the  Conductometer,  jepresented  in  Fig. 
211. 

The  conductometer  consists  of  a  circular  plate  of 
brass,  in  the  outer  edge  of  which  are  inserted  rods  of 
different  metals,  of  the  same  size  and  length,  each  hav- 
ing a  small  cavity  in  its  extremity  for  holding  a  piece 
of  phosphorus.  When  the  plate  is  brought  over  the 

flame  of  a  lamp,  the  heat  passes  along  the  different  rods  TnB  CONDUCTOMETEB. 
and  ignites  the  pieces  of  phosphorus,  but  not  all  at  the 

same  time.  It  first  reaches  the  end  of  the  rod  that  is  the  best  conductor ; 
and  thus  the  order  in  which  the  pieces  of  phosphorus  take  fire  indicates  the 
order  in  which  the  metals  that  the  rods  are  made  of  rank  as  conductors  of 
heat. 

491.  Conducting  Power  of  different  Substances. — Gold 
is  the  best  conductor  among  the  metals.     The  conducting 
power  of  gold  being  set  down  at  1,000,  that  of  some  other 
common  substances  compares  with  it  as  follows : — 


Platinum 981 

Silver 973 

Copper 898 


Iron 374    I    Lead 180 

Zinc 363         Marble 24 

Tin 304    I    Clay 11 


Platinum  and  silver,  it  will  be  seen,  are  nearly  as  good  conductors  as  gold. 

aach.  489.  What  are  Conductors  of  heat  ?  What  are  Bad  Conductors,  or  Non-con- 
ductors ?  As  a  general  rule,  what  substances  are  good  conductors  of  heat,  and  what 
Hot?  490.  How  do  the  metals  rank  in  conducting  power?  Describe  the  Con- 
ductometer, and  its  mode  of  operation.  491.  Among  the  metals,  what  is  the  best 
conductor?  The  next?  The  next?  Which  is  the  better,  iron  or  lead ?  How  may 


200  PYRONOMICS. 

A  silver  spoon  containing  water,  with  a  piece  of  muslin  wrapped  smoothly 
around  it,  may  be  held  in  the  flame  of  a  lamp  till  the  water  boils  without  the 
muslin's  burning,  so  rapidly  does  the  metal  carry  off  the  heat. 

492.  Wood  is_a  bad  conductor  of  heat.    A  log  blazing  at  one  end  may  be 
handled  at  the  other  without  inconvenience.    Hence  metallic  tea-pots,  sauce- 
pans, &c.,  are  often  provided  with  wooden  handles.     Dense  wood  and  coal 
are  better  conductors  than  porous  wood.    This  is  one  reason  why  they  are 
harder  to  kindle ;  they  conduct  the  heat  away  before  a  sufficient  amount  is 
collected  in  them  to  produce  combustion.    Earthen-ware  of  all  kinds  ranks 
far  below  the  metals  in  conducting  power. 

493.  Fibrous   substances,  like  wool,  hair,  and  fur,  are  bad  conductors. 
The  finer  and  closer  their  fibres,  the  less  their  conducting  power.    Thus  we 
see  why  Providence  has  clothed  the  animals  of  cold  climates  with  a  shaggy 
covering,  from  which  those  of  the  tropics  are  free ;  and  why  the  coats  of 
many  animals  in  temperate  regions  change  with  the  seasons,  being  closer  and 
longer  in  winter,  thinner  and  shorter  in  summer. 

494.  The  best  non-conductors  among  solids  are  straw,  saw-dust,  pow- 
dered charcoal,  and  plaster  of  paris.    Recourse  is  had  to  these  articles  when 
it  is  desired  to  protect  an  object  from  extremes  of  temperature.    Straw  is 
bound  round  tender  plants  in  winter,  to  prevent  their  warmth  from  being 
drawn  off.     It  is  also  used  for  thatching  the  roofs  of  houses,  preventing  the 
external  heat  from  entering  in  summer,  and  the  heat  within  from  being  with- 
drawn in  winter.     Ice  shipped  to  warm  climates  is  packed  in  saw -dust,  to 
keep  out  the  heat  of  the  atmosphere.    For  the  same  reason,  the  hollow  apart- 
ments that  constitute  the  sides  of  refrigerators  are  filled  with  powdered  char- 
coal.   Plaster  of  paris  is  used  for  filling  in  the  sides  of  fire-proof  safes.    So 
impervious  to  heat  does  it  render  them  that  they  may  be  exposed  to  flames 
for  hours  without  injury  to  the  papers  within. 

495.  If  we  bare  our  feet,  and  place  one  of  them  on  a 
carpet  and  the  other  on  oil-cloth,  the  latter  feels  much 
colder  than  the  former.  This  is  not  because  the  oil-cloth 
is  colder  than  the  carpet,  for  being  in  the  same  room  their 
temperature  must  be  the  same  ;  but  oil-cloth  is  a  good  con- 
ductor, whereas  carpet  is  not.  A  good  conductor,  brought 
in  contact  with  the  body,  carries  off  our  animal  heat  and 
makes  us  feel  cold.  A  bad  conductor,  on  the  other  hand, 
prevents  our  animal  heat  from  escaping.  Hence  the  differ- 

the  conducting  power  of  silver  be  proved  ?  492.  Why  are  metallic  tea-pots  often  pro- 
vided with  wooden  handles  ?  Why  is  dense  wood  hard  to  kindle  ?  How  does  earthen- 
ware rank  in  conducting  power?  493.  How  do  fibrous  substances  rank?  As  re- 
gards the  coats  of  animals,  how  is  the  goodness  of  Providence  shown?  494.  What 
are  the  best  solid  non-conductors  ?  For  what  are  these  substances  severally  used,  and 
what  is  the  effect  in  each  case  ?  495.  If  we  bare  our  feet,  and  place  one  on  a  carpet 
and  the  other  on  oil-cloth,  what  do  we  feel  ?  Explain  the  reason  of  this.  Of  tho 


DIFFUSION   OF  HEAT. 


201 


ence  of  warmth  in  different  kinds  of  clothing.  That  fabric 
feels  the  warmest,  which  is  the  worst  conductor. 

Of  the  materials  used  for  clothing,  wool  is  the  worst  conductor  and  linen 
the  best ;  cotton  and  silk  rank  between  the  two.  Linen  is  therefore  the  most 
comfortable  fabric  for  summer  clothing,  and  woollen  for  winter.  A  linen 
under-garment  is  cooler  than  a  silk  or  muslin  one,  and  these  in  turn  are 
much  cooler  than  flannel. 

496.  The  heat  of  our  bodies  is  generally  greater  than  that  of  the  atmos- 
phere surrounding  them.  If  we  were  placed  in  an  atmosphere  warmer  than 
our  bodies,  woollen  would  be  the  coolest  dress  that  could  be  worn,  because, 
being  a  bad  conductor,  it  would  not  transmit  the  external  heat.  Hence  fire- 
men and  others  exposed  to  a  high  degree  of  heat,  always  wear  flannel. 
Hence,  also,  a  blanket  is  wrapped  round  ice,  to  keep  it  from  melting. 

497.  Conducting  Power  of  Liquids.— Liquids  (except 
mercury,  which  is  a  metal)  are  very  bad  conductors  of  heat. 
This  may  be  shown  by  several  experiments. 

Freeze  some  water  in  the  bottom  of  a  tube,  Fig.  212. 

and  on  the  ice  pour  some  more  water.  Inclining 
the  tube,  apply  the  flame  of  a  lamp  to  the  liquid 
till  it  boils.  The  ice  remains  for  a  long  time  un- 
melted.  If  mercury  is  used  instead  of  water,  the 
ice  begins  to  melt  almost  immediately  on  the  ap- 
plication of  heat. 

Again,  in  a  funnel-shaped  glass  vessel  (repre- 
sented in  Fig.  212)  fix  a  thermometer,  or  instru- 
ment for  measuring  heat,  with  its  bulb  uppermost. 
Cover  the  bulb  with  water  to  the  depth  of  half  an 
inch  ;  then  pour  on  some  ether,  and  set  fire  to  it. 
The  burning  of  the  ether  generates  a  great  heat; 
yet  the  thermometer,  only  half  an  inch  below  it, 
indicates  little  or  no  increase  of  temperature. 

498.  Conducting  Power  of  Gases 
and  Vapors. — Gases  and  vapors   are 

still  worse  conductors  of  heat  than  liquids.  The  less  their 
specific  gravity,  the  less  appears  to  be  their  conducting 
power. 

499.  Air  is  one  of  the  worst  conductors  known.     If  we 

materials  used  for  clothing,  which  is  the  worst  conductor?  Which,  the  best  ?  How 
do  cotton  and  silk  rank  ?  What  fabric,  then,  is  the  most  appropriate  for  summer 
wear,  and  what  for  winter?  496.  Why  do  firemen  wear  flannel  ?  Why  is  a  blanket 
wrapped  round  ice  ?  497.  How  do  liquids  rank  in  conducting  power  ?  Prove  that 
water  is  a  bad  conductor.  Prove  it  by  an  experiment  with  the  apparatus  represent- 
od  in  Fig.  212.  498.  How  do  gases  and  vapors  rank  in  conducting  power?  499.  What 

9* 


202  PYRONOMICS. 

could  keep  a  body  of  air  perfectly  still,  it  would  take  a  long 
time  for  heat  applied  to  one  portion  of  it  to  be  transmitted 
throughout  the  whole. 

In  summer,  when  there  is  no  breeze,  we  feel  oppressively  warm,  because 
the  air  does  not  carry  off  the  heat  generated  within  us.  Fanning  cools  us, 
because  it  drives  off  the  air  heated  by  contact  with  our  bodies  and  brings  up 
a  fresh  supply,  which,  after  withdrawing  more  or  less  heat,  is  in  turn  driven 
away.  In  this  case  it  will  be  observed  that  the  heat  is  carried  off  by  convec- 
tion, and  not  by  conduction.  If  air  were  a  good  conductor,  it  would  soon 
take  so  much  heat  from  animals  and  plants  that  their  vital  action  could  not 
make  up  the  deficiency,  and  they  would  be  chilled  to  death. 

Closed  cellars  are  cooler  than  the  surrounding  air  in  summer,  and  warm- 
er in  winter.  If  air  were  a  good  conductor,  this  would  not  be  the  case.  As 
it  is,  the  doors  being  kept  closed,  currents  of  air  are  excluded ;  and,  since 
heat  passes  very  slowly  from  particle  to  particle,  extremes  of  temperature 
without  are  not  felt  within. 

It  is  the  air  in  fibrous  and  porous  solids  that  makes  them  bad  conductors. 
Drive  out  this  air  by  compression,  and  you  increase  their  conducting  power. 
Let  wool,  or  cotton,  for  instance,  be  twisted  into  rolls,  and  it  will  carry  off 
heat  faster  than  it  did  when  loose.  Accordingly,  clothing  that  allows  some 
air  to  remain  in  contact  with  the  body  is  warmer  than  that  which  fits  very 
tight.  So,  double  sashes  and  double  doors,  confining  a  body  of  non-con- 
ducting air,  protect  apartments  from  extremes  of  heat  and  cold. 

500.  The  uses  of  air  as  a  non-conductor  are  seen  in  the  operations  of  na- 
ture. Filling  the  pores  and  interstices  in  the  bark  of  plants,  it  protects  the 
tender  parts  within  from  sudden  falls  of  temperature.  In  cold  climates,  vege- 
tation is  further  protected  by  snow,  which,  owing  to  the  air  imprisoned 
among  its  particles,  is  a  very  bad  conductor.  A  mantle  of  snow  on  a  field 
has  very  much  the  same  effect  that  a  covering  of  wool  would  have.  Hence 
we  are  told  in  Scripture  that  God  "  giveth  snow  like  wool". — The  Esquimaux 
shield  themselves  from  the  excessive  cold  of  their  climate  in  huts  of  snow. 

501.  CONVECTION. — Fluids,  as  we  have  just  seen,  are 
bad  conductors,  but  they  are  readily  heated  by  convection. 
Heat  being  applied  beneath,  the  lower  particles  become 
expanded  and  rarefied.  They  therefore  ascend,  carrying 
up  their  heat,  while  cooler  and  heavier  particles  from  above 

is  said  of  the  conducting  power  of  air?  Why  do  we  feel  oppressively  warm  in  sum- 
mer, when  there  is  no  breeze  ?  What  is  the  effect  of  fanning  ?  If  air  were  a  good 
conductor,  what  would  be  the  consequence  to  animals  and  plants  ?  Why  are  closed 
cellars  exempt  from  extremes  of  temperature  ?  What  makes  fibrous  and  porous  sol- 
ids bad  conductors?  Prove  this.  Compare  the  warmth  of  loose  clothing  with  that 
which  fits  very  tight.  On  what  principle  do  double  sashes  operate  ?  500.  Show  th« 
Uses  of  air  as  a  non-conductor  in  the  economy  of  nature.  What  is  the  effect  of  snow  ? 
What  use  is  made  of  it  by  the  Esquimaux?  501.  How  are  fluids  readily  heated? 


DIFFUSION   OF   HEAT.  203 

take  their  place.  This  process  is  repeated  till  heat  is  dif- 
fused throughout  the  whole, — not  conducted  from  one  sta- 
tionary particle  to  another,  but  actually  conveyed  by  the 
particles  receiving  it. 

The  process  of  convection  is  exhibited  when  water  is  set  over  a  fire  to 
boil.  The  particles  soon  begin  to  move,  as  may  be  shown  by  throwing  in 
some  powdered  amber,  which  is  seen  to  rise  and  descend,  more  and  more 
rapidly  as  the  temperature  increases.  Heat  is  thus  diffused  throughout  the 
whole  body  of  liquid,  till  ebullition,  or  boiling,  commences. 

502.  In  cooling,  this  process  is  reversed.    The  particles  at  the  top  yield 
their  heat  to  the  air  in  contact  with  them.    Being  thus  made  heavier,  they 
descend,  while  warmer  and  lighter  particles  take  their  place.    The  greater 
the  surface  exposed  to  the  air,  the  sooner  the  liquid  loses  its  heat ;  hence  we 
pour  our  tea  into  a  saucer,  to  cool  it. 

503.  To  heat  a  body  of  liquid  by  convection,  the  fire  must  be  applied  be- 
neath.   A  pot  of  water  can  not  be  made  to  boil  by  a  fire  kindled  on  its  lid. 
The  particles  at  the  top  may  be  heated,  but  they  will  remain  there  on  ac- 
count of  their  superior  lightness,  and  there  will  be  no  diffusion  of  heat. 

504.  Thin  liquids,  like  water,  are  heated  and  cooled  more  quickly  than 
thick  ones,  like  tar,  because  their  particles  move  more  freely  among  them- 
selves, and  thus  diffuse  heat  more  readily. 

505.  Heat   is  diffused  through  gases   and  vapors,  as 
through  liquids,  by  convection.     Heated  air,  like  heated 
water,  ascends,  carrying  its  heat  with  it.     Consequently,  to 
make  the  temperature  of  a  room  uniform,  a  fire-place  should 
be  set  as  near  the  floor  as  possible. — With  the  same  tem- 
perature, we  feel  colder  on  a  windy  day  than  on  a  still  one  ; 
because  the  heat  is  more  rapidly  withdrawn  from  our  bodies 
by  the  fresh  currents  of  air  constantly  brought  in  contact 
with  them. 

506.  Solids  can  not  be  heated  by  convection,  because 
their  particles  cohere. 

507.  RADIATION. — A  body  not  in  contact  with  the  source 
of  heat  can  not  be  heated  by  conduction  or  convection.    If 
it  receives  heat,  it  is  by  a  third  process,  called  Radiation. 

Describe  the  operation.  In  what  familiar  process  is  convection  exhibited  ?  Describe 
the  process  of  boiling.  502.  Describe  the  process  of  cooling.  503.  To  heat  a  liquid, 
where  must  the  fire  be  applied  ?  Why  can  not  a  pot  of  water  be  made  to  boil  by  a  fire 
kindled  on  its  lid  ?  504  What  kind  of  liquids  are  heated  and  cooled  most  quickly  ? 
Why  ?  505.  What,  besides  liquids,  are  heated  by  convection  ?  Where  should  a  fire- 
place be  set,  and  why  ?  Why  do  we  feel  colder  on  a  windy  day  than  on  a  still  one  ? 
506.  Can  solids  be  heated  by  convection  ?  Why  not  ?  507.  What  bodies  are  heated 


204  PYBONOMICS. 

If  we  place  our  hands  under  a  fire  in  a  grate,  we  at  once  feel  a  sensation 
of  heat.  This  heat  can  not  reach  our  hands  by  conducaon,  for  air  is  a  bad 
conductor, — nor  by  convection,  for  heated  currents  ascend.  It  is  transmitted 
in  rays  sent  forth  from  the  fire  through  the  intervening  space.  Heat  thus 
diffused  is  called  Radiant  Heat.  All  the  heat  that  we  receive  from  the  sun, 
and  much  of  that  from  fire,  is  radiant  heat. 

508.  All  substances  radiate  heat,  but  not  equally  well. 
Much  depends  on  the  character  of  the  surface.  Rough  and 
dull  surfaces  radiate  "better  than  smooth  and  bright  ones. 

Lamp-black  is  the  best  radiator  known.  Rating  its  ra- 
diating power  at  100,  that  of  crown-glass  is  90  ;  black  lead, 
75 ;  tarnished  lead,  45  ;  clean  lead,  19  ;  bright  metals  gen- 
erally, 12.  The  radiating  power  of  metals  is  increased  by 
scratching  their  surface,  or  letting  them  become  tarnished. 

509.  A  heated  body  confined  in  a  covered  vessel  parts  with  its  heat  more 
or  less  rapidly  according  to  the  radiating  power  of  the  vessel  containing  it. 
For  tea-pots,  therefore,  bright  silver  is  preferable  to  earthen-ware,  because  it 
is  a  worse  radiator  and  keeps  the  tea  warm  for  a  longer  time.  Stoves,  on 
the  contrary,  should  be  made  of  a  good  radiator,  so  that  the'  heat  of  the  fire 
may  be  freely  diffused.  Cast-iron  is  better  for  this  purpose  than  sheet-iron, 
because  its  surface  is  rough ;  the  radiating  power  of  both  is  increased  by 
rubbing  in  black  lead.  "When  heat  is  to  be  conveyed  from  one  room  to  an- 
other, a  pipe  should  be  used  of  bright  tin,  which  is  a  bad  radiator  and  pre- 
vents the  escape  of  heat  by  the  way. 

The  atmosphere  receives  its  heat,  not  directly  from  the  sun,  but  by  radia- 
tion from  the  earth ;  hence,  as  we  ascend  from  the  earth's  surface,  the  heat 
diminishes. 

510.  Law  of  Radiant  Seat. — Radiant  heat  diminishes 
in  intensity  as  the  square  of  the  distance  from  the  radiating 
body  increases. 

A  body  10  feet  from  a  fire  will  receive  from  it  only  l]loo  of  the  heat  that  a 
body  1  foot  from  it  receives. 

511.  Radiant  heat,  striking  different  bodies,  is  reflected 

by  radiation?  What  is  heat  diffused  by  radiation  called?  Give  a  familiar  example 
of  radiant  heat  508.  By  what  is  a  body's  radiating  power  affected  ?  What  surfaces 
ladiate  heat  the  best?  What  is  the  best  known  radiat*r?  Eating  the  radiating 
power  of  lamp-black  at  100,  what  is  that  of  crown-glass  ?  Black  lead  ?  Tarnished 
lead  ?  Clean  lead  ?  Bright  metals  generally  ?  How  may  the  radiating  power  of  the 
metals  be  increased  ?  509.  Why  is  bright  silver  preferable  to  earthen-ware  for  tea- 
pots ?  Of  what  should  stoves  be  made  ?  When  heat  is  to  be  conveyed  from  one  room 
to  another,  what  should  be  employed  ?  Why  ?  How  does  the  atmosphere  receiva 
Hs  heat?  What  follows?  510.  State  the  law  of  radiant  heat  Give  an  example. 


DIFFUSION   OF   HEAT. 


205 


"by  some,  absorbed  by  others,  and  transmitted  by  a  third 
class. 

512.  Reflection  of  Radiant  Heat. — Radiant  heat  is  re- 
flected by  polished  and  light-colored  surfaces.  Polished 
gold  reflects  about  three-fourths  of  the  radiant  heat  it  re- 
ceives, and  looking-glass  about  one-fifth  ;  whereas  metallic 
surfaces  blackened  reflect  only  one-twentieth. 

513.  White  and  light-colored  clothes  are  worn  in  summer,  because  they 
reflect  heat.    For  the  same  reason,  it  is  harder  to  heat  water  in  a  new  tin  ves- 
sel than  in  one  that  has  been  blackened  over  the  fire. 

514.  The  reflection  of  radiant  heat  may  be  illustrated  with  the  apparatus 
represented  in  Fig.  213.    A  and  B  are  concave  metallic  mirrors,  highly  pol- 

Fig.  213. 


ished.  In  the  focus  of  A  is  placed  a  red-hot  ball  C.  This  ball  radiates  hea* 
in  all  directions,  and  some  of  its  rays  strike  the  mirror  A,  from  which  they 
are  reflected  in  parallel  lines  to  B.  By  B  they  are  again  reflected  and  brought 
to  a  focus  at  D,  where  a  thermometer  indicates  a  rise  of  temperature.  Suffi- 
cient heat  may  thus  be  concentrated  at  D  to  set  fire  to  phosphorus  or  gun* 
powder. 

515.  When  radiant  heat  is  reflected  by  a  plane  surface, 
the  angle  of  reflection  (see  §  96)  is  always  equal  to  the  an- 
gle of  incidence.  If  it  strikes  the  surface  perpendicularly, 
it  is  reflected  perpendicularly,  back  to  the  radiating  body. 
If  the  line  in  which  it  approaches  the  surface  forms  an  angle 

611.  When  radiant  heat  strikes  different  bodies,  what  becomes  of  it?  512.  By  what 
rurfaces  is  radiant  heat  reflected  ?  What  portion  does  polished  gold  reflect  ?  Look- 
ing-jrlass  ?  Metallic  surfaces  blackened  ?  513.  Why  ar«  light-adored  clothes  worn  in 
summer  ?  In  what  sort  of  a  vessel  is  it  hardest  to  heat  water  ?  514.  Illustrate  tha 
reflection  of  radiant  heat  with  Fig.  213.  How  much  heat  may  be  concentrated  witlj 
this  apparatus  ?  515,  When  radiant  heat  is  reflected,  to  what  is  the  angle  of  reflection 


206  PYBONOMICS. 

with  the  perpendicular,  it  glances  off  at  an  equal  angle  on 
the  other  side. 

516.  Absorption  of  Radiant  Heat. — Radiant  heat  is 
absorbed  by  dull  and  dark-colored  surfaces.     Good  reflec- 
tors are  bad  absorbents  and  radiators ;  bad  reflectors  are 
good  absorbents  and  radiators. 

Of  the  colors,  black  is  the  best  absorbent  of  heat,  and 
violet  the  next  best ;  white  is  the  worst,  and  yellow  next  to 
the  worst. 

Lay  two  pieces  of  cloth,  one  white  and  the  other  black,  on  a  snow-bank, 
in  the  sunshine.  Under  the  black  piece,  which  absorbs  the  heat  that  strikes 
it,  the  snow  melts  rapidly ;  not  so  under  the  white  cloth,  for  by  it  the  heat  is 
reflected.  Dark-colored  clothing  is  therefore  best  adapted  to  winter. 

Dark  mould  absorbs  the  sun's  heat ;  hence  one  cause  of  its  fertility. 
White  sand  reflects  the  hot  rays ;  hence  it  burns  our  faces  when  we  walk 
over  it  in  summer.  Hoar-frost  remains  longer  in  the  morning  on  light  than 
dark  substances :  this  is  because  light  colors  reflect  the  sun's  heat,  while 
dark  colors  absorb  it,  and  thus  melt  the  hoar-frost,  which  is  nothing  more 
than  frozen  dew. 

517.  Transmission  of  Radiant  Heat. — Transparent  sub- 
stances, or  such  as  allow  light  to  pass  through  them,  for 
the  most  part  transmit  heat  also.     The  sun's  rays,  for  in- 
stance, falling  on  the  atmosphere  of  the  earth,  which  is  a 
transparent  medium,  are  transmitted  through  it  to  objects 
on  the  surface.     More  or  less  heat  is  absorbed  in  the  act 
of  transmission. 

518.  Substances  that  transmit  heat  freely  are  called  Di- 
a-ther'-ma-nous.     Those  that  absorb  the  greater  part  and 
transmit  little  or  none  are  called  A-ther'-ma-nous. 

519.  All  transparent  substances  are  not  diathermanous.  Water,  for  ex- 
ample, which  offers  but  little  obstruction  to  rays  of  light,  intercepts  nearly 
all  the  heat  that  strikes  it.  Alum  is  another  instance  in  point. 

equal  ?  516.  By  what  surfaces  is  radiant  heat  absorbed  ?  What  is  said  of  good  reflect- 
ors ?  What,  of  bad  reflectors  ?  What  color  is  the  best  absorbent  of  heat  ?  W  hat,  the 
next  best?  What  color  is  the  worst  absorbent?  What,  the  next  worst  ?  Prove  by 
an  experiment  the  difference  in  absorbing  power  between  white  and  black.  Why  ii 
'dark-colored  clothing  best  adapted  to  winter  ?  What  is  the  difference  between  dart 
mould  and  white  sand  in  absorbing  power?  Why  does  hoar-frost  remain  longer  in 
the  morning  on  light  than  dark  substances  ?  517.  What  substances,  for  the  most  part, 
transmit  heat  ?  Give  an  example.  518.  What  are  Diathermanous  substances  ?  What 
are  Athermanous  substances  ?  519.  Name  a  transparent  substance  that  is  not  dia- 


EFFECTS   OF   HEAT.  207 

All  diathermanous  substances  are  not  transparent.  Quartz,  though  it  may 
intercept  light  almost  entirely,  transmits  heat  quite  freely. 

As  a  general  rule,  the  rarer  transparent  substances,  such  as  gases  and 
vapors,  transmit  heat  the  best ;  the  denser  ones,  such  as  rock-crystal,  trans- 
mit it  the  least  freely.  The  farther  the  rays  have  to  pass  through  a  given 
substance,  the  more  heat  is  intercepted. 

Effects  of  Heat. 

520.  The  effects  of  heat  are  five  in  number :  Expansion, 
which  changes  the  size  of  bodies  ;  Liquefaction  and  Vapor- 
ization, which  change  their  form;  Incandescence,  which 
changes  their  color ;  and  Combustion,  which  changes  their 
nature. 

521.  EXPANSION. — Heat  expands  bodies. 

Insinuating  itself  between  the  particles  of  bodies,  it  forces  them  asunder, 
and  thus  makes  them  occupy  a  greater  space.  Heat,  therefore,  opposes  co- 
hesion. Solids,  in  which  cohesion  is  strongest,  expand  the  least  under  the 
influence  of  heat ;  liquids,  having  less  cohesion,  expand  more ;  gases  and 
vapors,  in  which  cohesion  is  entirely  wanting,  expand  the  most.  Heat  con- 
verts solids  into  liquids,  and  liquids  into  gases  and  vapors,  by  weakening 
their  cohesion.  It  turns  ice,  for  example,  into  water,  and  water  into  steam. 

522.  Expansion  of  Solids. — All  solids  except  clay  are 
expanded  by  heat ;  but  not  equally.     Of  the  metals,  tin  is 
among  those  that  expand  most.   Clay  is  contracted  by  bak- 
ing, and  ever  afterwards  remains  so  ;  this  is  supposed  to  be 
owing  to  a  chemical  change  produced  in  it  by  heat. 

The  expansion  of  solids  is  illustrated  with  the  apparatus  represented  in 
Fig.  214.  A  brass  ball  is  suspended  from  a  pillar,  to  which  is  also  at- 
tached a  ring  just  large  enough  to  let  the  ball  pass  through  it  at  ordinary 
temperatures.  Heat  the  ball  with  a  lamp  placed  beneath,  and  it  will  ex- 
pand to  such  a  degree  that  it  can  not  pass  through  the  ring.  Let  it  cool,  and 
it  will  go  through  as  before. 

523.  A  sheet-iron  stove  in  which  a  hot  fire  is  quickly  kindled  or  put 
out,  sometimes  makes  a  cracking  noise,  in  consequence  of  the  rapid  ex- 

thermanous.  Name  a  diathermanous  substance  that  is  not  transparent.  As  a  gen- 
eral rule,  what  transparent  substances  transmit  heat  the  best,  and  what  the  worst? 
520.  State  the  effects  of  heat.  521.  What  is  the  first  of  these  ?  How  is  it  that  heat 
expands  bodies  ?  What  force  does  it  oppose  ?  Which  expand  the  most  under  the 
influence  of  heat,  solids,  liquids,  or  gases,— and  why  ?  Into  what  does  heat  convert 
solids  ?  Into  what,  liquids  ?  522.  What  solids  are  expanded  by  heat  ?  What  metal 
is  expanded  more  than  most  of  the  others  ?  What  is  the  effect  of  heat  on  clay  ?  II- 
Instrate  the  expansion  of  solids  with  the  apparatus  represented  in  Fig.  214.  523.  Why 


208 


PYKONOMICS. 


Fig.  214.  pansion  or  contraction  of  the  metal.    A  blower 

placed  on  or  taken  from  a  hot  fire  produces  a  sim- 
ilar noise  for  the  same  reason.  New  furniture 
standing  in  the  sun  or  near  a  fire  is  apt  to  warp 
and  crack  in  consequence  of  the  expansive  effects 
of  heat. 

When  boiling  water  is  poured  into  china  cups 
and  glass  vessels,  they  often  crack.  This  is  be- 
cause the  inner  surface  is  expanded  by  heat» 
while  the  outer  is  not,  china-ware  and  glass  be- 
ing bad  conductors.  The  unequal  expansion 
cracks  the  vessel.  Cold  water  poured  on  a  hot 
glass  or  stove  produces  the  same  effect.  On  the 
same  principle,  glass  chimneys  are  apt  to  crack, 
when  brought  too  suddenly  over  the  flame  of  a 
lamp  or  gas-burner.  A  cut  made  in  the  bottom 
with  a  diamond  allows  an  opportunity  for  expan- 
sion, and  prevents  the  chimney  from  breaking. 

When  a  glass  stopper  becomes  fastened  in  a  bottle,  it  may  often  be  with, 
drawn  by  placing  the  neck  of  the  bottle  in  warm  water.  The  neck  is  ex- 
panded before  the  heat  reaches  the  stopper. 

524.  The  force  with  which  a  body  expands  when  heat- 
ed and  contracts  when  cooling,  is  very  great.  In  iron 
bridges,  therefore,  and  other  structures  in  which  long  bars 
of  metal  are  employed,  there  is  danger  of  the  parts'  sep- 
arating, unless  provision  is  made  for  the  expansion  caused 
by  a  rise  of  temperature.  The  middle  arch  of  an  iron 
bridge  has  been  known  to  rise  an  inch  in  the  summer  of  a 
temperate  climate.  So,  when  great  lengths  of  iron  pipe 
are  laid  for  conveying  steam  or  hot  water,  sliding  joints 
must  be  used,  or  the  apparatus  will  burst  in  consequence 
of  the  expansion  of  the  metal. 

525.  The  fact  that  heat  expands  bodies  and  cold  contracts  them,  is  often 
turned  to  practical  account.  Coopers,  for  instance,  heat  their  iron  hoops, 
and  while  they  are  thus  expanded  put  them  on  casks  which  they  just 
fit.  As  they  cool,  they  contract  and  bind  the  staves  tightly  together.  The 


do  a  sheet-iron  stove  and  a  blower  sometimes  make  a  cracking  noise?  What  causes 
new  furniture  to  warp  ?  What  makes  glass  vessels  crack  when  boiling  water  is  poured 
into  them?  When  are  glass  Chimneys  apt  to  crack?  How  may  their  cracking  be 
prevented  ?  When  a  glass  stopper  becomes  fastened  in  a  bottle,  how  may  it  be  with- 
drawn? 524  What  is  said  of  the  force  with  which  bodies  expand  and  contract? 
What  precautions  must  be  taken  in  consequence  ?  525.  What  practical  use  is  made 
of  the  fact  that  heat  expands  bodies  and  cold  contracts  them  ?  What  ingenious  appU- 


EXPANSION.  209 

wheel-wright  fastens  the  tire,  or  outer  rim  of  iron,  on  his  wheel  in  the 
same  way. 

The  contraction  of  iron,  when  cooling,  has  been  ingeniously  used  for 
drawing  together  the  walls  of  buildings  that  have  bulged  out  and  threaten 
to  fall.  Several  holes  are  made  opposite  to  each  other  in  the  walls,  into 
which  are  introduced  stout  bars  of  iron,  projecting  on  both  sides  and  termi- 
nating at  each  end  in  a  screw.  To  each  screw  a  nut  is  fitted.  The  bars  are 
then  heated  by  lamps  placed  beneath,  and  when  they  have  expanded  the 
nuts  are  screwed  up  close  to  the  walls.  As  the  bars  cool,  they  gradually  con- 
tract, and  with  such  force  as  to  bring  the  walls  back  to  a  perpendicular  po- 
sition. 

526.  Expansion  of  Liquids.— Liquids,  when  heated, 
expand  much  more  than  solids,  but  not  all  alike.     Thus 
water,  raised  from  its  freezing-point  to  the  temperature  at 
which  it  boils,  has  its  bulk  increased  one-twenty-second ; 
alcohol,  between  the  same  limits,  increases  one-ninth. 

The  higher  the  temperature,  the  greater  the  rate  at 
which  liquids  expand. 

527.  In  proportion  as  heat  expands  liquids,  it  rarefies 
them,  the  same  quantity  of  matter  being  made  to  occupy 
a  larger  space.     This  fact  is  shown  in  the  process  of  boil- 
ing, described  in  §  501. 

528.  Water  at  certain  temperatures  forms  a  remarkable 
exception  to  the  general  law  that  liquids  are  expanded  by 
heat  and  contracted  by  cold.     As  it  cools  down  from  the 
boiling-point,  it  contracts,  and  consequently  increases  in 
density,  till  it  reaches  39  degrees,  or  7  degrees  above  its 
freezing-point.     Below  this  temperature,  it  expands. 

The  expansion  of  water  in  freezing  is  proved  every  winter  by  the  burst- 
ing of  pipes,  pitchers,  &c.,  containing  it.  The  force  with  which  it  expands 
is  tremendous.  An  iron  plug  weighing  three  pounds  and  closing  a  bomb- 
shell filled  with  water,  has  been  thrown  15  feet  by  the  freezing  and  expansion 
of  the  liquid  within.  Immense  masses  of  rock  are  sometimes  split  off  by  the 
freezing  of  water  which  has  insinuated  itself  into  minute  fissures. 

The  expansion  and  consequent  rarefaction  of  water  in  freezing,  afford  a 

cation  has  been  made  of  the  contraction  of  iron  -when  cooling?  Give  an  account  of 
the  process.  526.  How  does  the  expansion  of  heated  liquids  compare  with  that  of 
eolids?  Compare  the  expansion  of  water  with  that  of  alcohol.  On  what  does  the  rate 
at  which  liquids  expand  depend  ?  527.  Besides  expanding  liquids,  what  does  heat  do 
to  them?  528.  What  exception  is  there  to  the  law  that  liquids  are  contracted  by 
cold  ?  How  is  the  expansion  of  water  in  freezing  proved  ?  What  cases  are  cited,  to 
show  the  great  force  with  which  water  expands  in  freezing  ?  How  does  the  expansion 


210  PYRONOMICS. 

•trikiag  proof  of  the  goodness  of  Providence.  The  great  body  of  a  large 
mass  of  water  never  becomes  cold  enough  to  freeze  ;  it  freezes  only  011  the 
top,  where  it  comes  in  contact  with  very  cold  air.  As  it  is,  the  ice  formed 
on  the  surface  remains  there  on  account  of  its  superior  rarity,  and  protects 
the  water  below  and  the  fish  that  inhabit  it  from  further  cold.  If  water  con- 
tinued to  contract  and  increase  in  density  as  it  approached  the  freezing-point, 
the  ice  first  formed  would  sink ;  the  fresh  surface  exposed  to  the  air  would 
in  its  turn  freeze,  and  another  layer  of  ice  would  sink ;  and  this  would  go  on 
till  even  in  a  mild  winter  every  body  of  water  would  be  converted  into  a  solid 
mass,  and  all  living  things  therein  destroyed. 

529.  Iron,  zinc,  and  several  other  metals,  when  cooling  down  from  a  melt- 
ed to  a  solid  state,  expand  like  freezing  water.  This  is  because  the  particles 
assume  a  crystalline  arrangement,  by  which  greater  interstices  are  left  be- 
tween them. 

530.  Expansion  of  Gases  and  Vapors. — Aeriform  bodies 
expand  equally  under  a  given  increase  of  temperature.  At 
the  boiling-point  of  water,  their  bulk  is  one-third  greater 
than  at  the  freezing-point. 

531.  Fill  a  bladder  with  air,  tie  its  neck,  and  place  it  before  a  fire ;  the 
heat  will  soon  expand  the  confined  air  to  such  a  degree  as  to  burst  the 
bladder. 

The  popping  of  grains  of  corn,  the  bursting  open  of  chestnuts  when 
roasting,  and  the  crackling  of  burning  wood,  are  caused  by  the  expansion 
of  the  air  within  them.  Porter-bottles  have  to  be  kept  in  a  cool  place  in 
summer,  lest  the  heat  expand  the  carbonic  acid  in  the  porter  and  break  the 
bottles. 

532.  LIQUEFACTION. — Heat  melts  solids.     This  process 
is  called  Liquefaction. 

Some  solids,  such  as  wax  and  butter,  require  but  little  heat  to  melt  them. 
Others,  like  metals  and  stones,  melt  only  at  the  highest  temperatures  that 
can  be  produced.  Such  substances  are  called  refractory. 

Even  substances  that  are  liquid  at  ordinary  temperatures  may  be  looked 
upon  as  melted  solids,  for  they  can  be  reduced  by  cold  to  the  solid  state. 

533.  When  a  solid  is  converted  into  a  liquid,  sensible 
heat  is  absorbed.     When  a  liquid  is  converted  into  a  solid, 


of  water  in  freezing  exhibit  the  goodness  of  Providence  ?  529.  How  do  we  account 
for  the  expansion  of  several  of  the  metals,  when  cooling  down  from  a  melted  state  ? 
630.  "What  is  said  of  the  expansion  of  aeriform  bodies  ?  How  great  is  their  expansion, 
when  they  are  raised  from  the  freezing-point  to  the  boiling-point  of  water  ?  531.  How 
may  we  illustrate  the  expansion  of  air  by  heat  with  a  bladder  ?  What  familiar  exam- 
ples are  given  of  the  expansion  of  air  by  heat  ?  532.  What  is  Liquefaction  ?  What 
difference  is  there  in  solids,  as  regards  their  capability  of  being  melted  ?  How  may 
•instances  that  are  liquid  at  ordinary  temperatures  be  looked  upon?  583.  By  what 


VAPORIZATION.  211 

latent  heat  is  given  out.  This  is  another  merciful  provision, 
for  thus  extremes  of  temperature  and  their  effects  are  mod- 
ified. 

When  a  solid  is  rapidly  melted,  so  much  heat  is  absorbed  by  the  liquid 
that  intense  cold  is  produced.  This  is  the  principle  on  which  freezing  mix- 
tures operate.  Ice  cream,  for  instance,  is  frozen  with  a  mixture  of  salt  and 
snow  or  pounded  ice ;  the  latter  is  rapidly  melted,  and  so  much  heat  is  ab- 
sorbed in  the  process  that  the  cream  is  brought  to  a  solid  form. 

534.  VAPORIZATION. — Heat  converts  liquids  into  vapors. 
This  process  is  called  Vaporization. 

Heat,  applied  to  a  solid,  first  expands  it,  then  melts  it,  and  finally  turns  it 
into  vapor.  Some  solids  pass  at  once  into  vapor,  without  becoming  liquids. 

535.  A  great  degree  of  heat  is  not  essential  to  vapori- 
zation.    At  ordinary  temperatures,  wherever  a  surface  of 
water  is  in  contact  with  the  air,  vapor  is  formed.     This  pro- 
cess is  known  as  Spontaneous  Evaporation.     By  its  means 
the  atmosphere  becomes  charged  with  moisture,  and  clogds 
and  dew  are  formed.     The  drier  the  air,  and  the  more  it  is 
agitated,  so  as  to  bring  fresh  currents  in  contact  with  the 
liquid,  the  more  rapidly  does  evaporation  take  place. 

536.  A  drop  of  water  let  fall  on  a  cold  iron  moistens  its  surface ;  let  fall 
on  a  very  hot  iron,  it  hisses  and  runs  off  without  leaving  any  trace  of  moist- 
ure. In  the  latter  case,  the  water  does  not  touch  the  iron  at  all,  but  is  sep- 
arated from  it  by  a  thin  layer  of  vapor  into  which  part  of  the  drop  is  con- 
verted by  the  heat  radiated  from  the  iron.  Laundresses  try  their  irons  in 
this  way,  to  see  if  they  are  hot  enough  for  use.  On  the  same  principle,  jug- 
glers plunge  their  hands  into  melted  metal  with  impunity,  by  first  wetting 
them.  The  moisture  on  their  hands  is  converted  into  vapor,  which  keeps  the 
seething  metal  from  their  skin. 

537.  When  vapor  is  formed,  sensible  heat  is  absorbed, 
and  cold  is  produced. 

Hence  when  the  skin  is  moistened  with  a  volatile  liquid  (that  is,  one  that 
readily  passes  into  vapor)  like  alcohol,  a  sensation  of  cold  is  soon  expe- 
rienced. So,  a  shower  or  water  sprinkled  on  the  floor  cools  the  air  in  sum- 
merciful  provisions  are  extremes  of  temperature  modified  ?  On  what  principle  do 
freezing  mixtures  operate  ?  584.  What  is  Vaporization  ?  What  are  the  successive 
effects  of  heat  on  solids  ?  535.  What  is  Spontaneous  Evaporation  ?  What  are  tho 
effects  of  evaporation  on  the  earth's  surface  ?  To  what  is  the  rapidity  of  evaporation 
proportioned?  536.  Explain  the  principle  on  which  laundresses  try  their  irons. 
"What  use  do  jugglers  make  of  this  principle?  537.  With  what  phenomena  is  the 
formation  of  vapor  accompanied  ?  Give  some  examples,  of  cold  produced  by  the  for- 


212 


PYKONOMICS. 


mer. — Green  wood  does  not  make  so  hot  a  fire  as  dry,  because,  when  the 
moisture  it  contains  is  converted  into  vapor,  a  large  amount  of  sensible  heat 
is  absorbed  and  carried  off. 

538.  CONDENSATION. — The  turning  of  vapor  back  into  a 
liquid  state  is  called  Condensation. 

539.  Distillation. — Some  substances  are  converted  into 
vapor  at   lower  temperatures   than  others.      This  fact  is 
taken  advantage  of  in  Distillation. 

Distillation  is  the  process  of  separating  one  substance 
from  another  by  evaporating  and  then  condensing  it.  It 
was  known  to  the  Arabians  at  an  early  date.  Fig.  215 
represents  a  Still,  or  apparatus  for  distilling. 

Fig.  215. 


540.  A  is  a  boiler,  resting  on  a  furnace.  In  its  head,  B,  is  inserted  a  pipe, 
b  c,  which  enters  the  worm-tub,  R,  and  there  terminates  in  a  worm,  represented 
by  the  dotted  lines.  The  substance  to  be  distilled  having  been  placed  in  the 
boiler  and  a  fire  kindled  beneath,  vapor  soon  rises.  Passing  through  the 
pipe  b  c,  it  enters  the  worm,  in  which  it  is  to  be  condensed.  The  worm  is 
surrounded  with  cold  water,  with  which  the  vat  is  filled,  and  the  vapor  is 
soon  cooled  down  into  a  liquid  form,  and  issues  from  the  lower  extremity  of 

tnation  of  vapor.  "Which  makes  the  hotter  fire,  green  wood  or  dry,— and  why? 
638.  What  is  meant  by  the  Condensation  of  vapor  ?  539.  What  is  Distillation  ?  On 
what  fact  is  the  process  based  ?  To  whom  was  distillation  early  known  ?  What  is 
an  apparatus  for  distilling  called?  540.  With  the  aid  of  Fig.  215,  describe  the  still, 


INCANDESCENCE.  213 

the  worm,  falling  into  a  vessel  prepared  to  receive  it.  To  condense  the  va- 
por, the  water  in  the  vat  must  be  kept  cold.  For  this  purpose,  a  stream  is 
kept  flowing  into  it  through  the  pipe  pjy,  while  a  similar  stream  of  water 
partially  warmed  by  the  hot  vapor  as  constantly  escapes  at  q.  By  this  pro- 
cess water  may  be  obtained  perfectly  pure,  as  the  earthy  matter  dissolved  in 
it  is  not  converted  into  vapor,  but  remains  behind  in  the  boiler.  With  a 
similar  apparatus,  spirituous  liquors  are  distilled  from  grain. 

541.  INCANDESCENCE. — When  a  body  is  raised  to  a  cer- 
tain very  high  temperature,  it  begins  to  emit  light  as  well 
as  heat.     This  state  is  called  Incandescence,  or  Glowing 
Heat. 

An  incandescent  body  becomes  successively  dull  red, 
bright  red,  yellow,  and  white.  All  solids  and  liquids,  not 
previously  converted  into  vapor  by  heat,  become  incan- 
descent. The  temperature  at  which  incandescence  com- 
mences is  the  same  for  all  bodies,  and  may  be  set  down  at 
977  degrees  of  Fahrenheit's  Thermometer  (see  §  544). 

Instruments  for  measuring  Heat. 

542.  The  expansion  of  bodies  by  heat  furnishes  us  the 
means   of  measuring  changes   of  temperature.      Liquids, 
which  are  easily  affected,  are  used  for  measuring  variations 
in  moderate  temperatures.     Solids,  which  require  a  higher 
degree  of  heat  to  expand  them  perceptibly,  are  used  for 
measuring  variations  in  elevated  temperatures.     Hence  we 
have  two  instruments,  the  Thermometer  and  the  Pyrom- 
eter. 

543.  THE  THERMOMETER. — The  Thermometer  is  an  in- 
strument in  which  a  liquid,  usually  mercury,  is  employed 
for  measuring  variations  that  occur  in  moderate  tempera- 
tures. 

The  thermometer  (see  Fig.  216)  consists  of  a  tube  closed  at  one  end  and 
terminating  in  a  bulb  at  the  other.  The  bulb  and  part  of  the  tube  contain 
mercury,  above  which  is  a  vacuum,  all  air  having  'been  excluded  before  the 
top  of  the  tube  was  closed.  Expanded  by  heat,  the  mercury  rises  in  the 

and  its  mode  of  operation.  541.  What  is  Incandescence  ?  What  colors  mark  the 
successive  stages  of  incandescence?  What  substances  become  incandescent?  At 
what  temperature  does  incandescence  commence?  642.  What  means  have  we  of 
measuring  changes  of  temperature  ?  In  what  cases  are  liquids  used  ?  In  what,  sol- 
ids? Name  the  instruments  used  for  measuring  changes  of  temperature.  643.  What 


214 


PTEONOMICS. 


Fig.  216. 


tube ;  when  the  temperature  falls,  the  mercury,  contracting, 
falls  also.  The  tube  is  fixed  in  a  stand  or  case,  and  has  a 
graduated  scale  beside  it  for  measuring  the  rise  and  fall  of  the 
mercury.  This  scale  is  formed  in  the  following  way  : — The  ther- 
mometer is  brought  into  contact  with  melting  ice,  and  the  point 
at  which  the  mercury  stands  is  marked.  It  is  next  plunged 
in  boiling  water,  and  the  point  to  which  the  mercury  rises  is 
also  marked.  The  interval  is  then  divided  into  a  number  of 
equal  spaces,  called  degrees. 

544.  As  the  thermometer  does  not  indicate 
the  amount  of  heat  in  a  body,  but  merely  its 
changes  of  temperature,  the  number  of  degrees 
into  which  the  interval  between  the  freezing  and 
the  boiling  mark  is  divided  is  arbitrary.  Three 
different  divisions  are  in  use :  Fahrenheit's,  in 
the  United  States,  Great  Britain,  and  Holland ; 
Reaumur's  [ro'-murz],  in  Spain  and  parts  of  Ger- 
many; and  the  Centigrade,  the  most  convenient 
of  the  three,  in  France,  Sweden,  &c. 

In  Fahrenheit's  scale  the  freezing-point  is  called  32,  the 
boiling-point,  212 ;  when,  therefore,  the  mercury  stands  at  0, 
or  zero,  it  is  32  degrees  below  the  freezing-point.  In  Reau- 
mur's scale  the  freezing-point  is  called  0,  the  boiling-point  80. 
In  the  Centigrade  the  freezing-point  is  0,  the  boiling-point  100. 
When  degrees  of  the  thermometer  are  mentioned,  it  is  usual 
to  indicate  the  scale  referred  to  by  the  letters  P.,  R.,  or  C.,  as 
the  case  may  be.  Thus  40°  F.  means  40  degrees  on  Fahren- 
heit's scale ;  15°  R.,  15  degrees  on  Reaumur's  scale,  &c.  In  this  country, 
when  no  scale  is  mentioned,  Fahrenheit's  is  meant. 

545.  Imperfect  thermometers  were  in  use  at  the  beginning  of  the  seven- 
teenth century.  It  is  uncertain  whether  the  honor  of  their  invention  belongs 
to  Sanctorio,  an  Italian  physician, — Drebbel,  a  Dutch  peasant, — or  Galileo. 
Various  liquids  have  been  tried ;  the  astronomer  Roemer  was  the  first  to  use 
mercury,  the  advantages  of  which  are  such  that  it  has  superseded  all  other* 

546.   The  Differential  Thermometer. — This  instrument, 


THE  TIIER 
MOMETER. 


is  the  Thermometer?  Of  what  does  it  consist?  How  is  the  scale  of  the  thermome- 
ter formed  ?  544.  What  is  said  of  the  number  of  degrees  into  which  the  scale  is  di- 
vided? Name  the  three  principal  scales,  and  tell  where  each  is  used.  What  are  the 
freezing-point  and  the  boiling-point  respectively  called  in  Fahrenheit's  scale  ?  What, 
in  Reaumur's  scale  ?  In  the  Centigrade  scale  ?  How  are  the  different  scales  indi- 
cated ?  545.  When  were  thermometers  first  used  ?  To  whom  does  the  honor  of  their 
invention  belong?  What  liquid  has  superseded  all  others  in  the  thermometer  ?  Who 


THE  DIFFERENTIAL  THERMOMETER. 


215 


represented  in  Fig.  217,  measures  minute  dif- 
ferences of  temperature. 

It  consists  of  a  long  glass  tube,  bent  twice  at  right  an- 
gles, somewhat  in  the  form  of  the  letter  U.  One  arm  is 
furnished  with  a  scale  of  100  degrees,  and  each  terminates 
in  a  bulb.  The  tube  contains  a  small  quantity  of  sulphu- 
ric acid,  colored  red,  and  so  disposed  that  when  both 
bulbs  are  of  the  same  temperature  it  stands  at  0  on  the 
scale.  Let  either  bulb  be  heated  ever  so  little  more  than 
the  other,  and  the  expansion  of  the  air  within  will  drive 
the  liquid  down  and  cause  it  to  ascend  the  opposite  arm  to 
a  distance  measured  by  the  scale.  Ordinary  changes  of 
temperature  do  not  affect  the  instrument,  because  both 
bulbs  are  acted  on  alike. 

547.  THE  PYROMETER. — The  Pyrometer 
(see  Fig.  218)  is  used  for  measuring  variations 
in  elevated  temperatures,  and  comparing  the 
expansive  power  of  different  metals  for  a 
given  degree  of  heat. 

Fig.  218. 


Fig.  217. 


O 


THE  DIFFERENTIA!. 
THERMOMETER. 

A  metal  bar  is  fixed 
in  an  upright  at  one 
end  by  means  of  a 
screw,  and  left  free  to 
expand  at  the  other. 
It  there  touches  a  pin 
projecting  from  a  rod 
which  rests  against  an 
opposite  upright,  in  a 
circular  support  at 
each  side.  This  rod 

terminates  at  one  end  in  an  arm  bent  at  right  angles,  which  is  connected  by 
a  cord  and  pulley  with  an  index  traversing  a  scale  marked  with  degrees. 
Near  its  extremity  is  a  ball,  the  weight  of  which,  under  ordinary  circum- 
stances, keeps  the  index  at  the  highest  point  of  the  scale.  When  lamps  are 
placed  beneath  and  the  bar  expands,  it  pushes  against  the  pin,  turns  the  rod 

first  used  it  ?  546.  For  what  is  the  Differential  Thermometer  employed  ?  Describe 
the  differential  thermometer,  and  its  operation.  547.  For  what  is  the  Pyrometer 


THE   PYROMETER. 


216  PYEONOMICS. 

more  or  less  around,  and  thus  raises  the  arm  containing  the  ball  and  moves 
the  index  along  the  scale.  The  relative  degree  of  heat  applied  to  the  bar  is 
thus  indicated.-  By  keeping  the  heat  the  same,  and  using  rods  of  different 
metals,  we  can  ascertain  their  relative  expansive  power. 

Specific  Meat. 

548.  Put  a  pound  of  water  and  a  pound  of  olive  oil  in 
two  similar  vessels,  and  apply  heat.  It  will  take  twice  as 
long  to  raise  the  water  to  a  given  temperature  as  it  will  the 
oil.  Let  them  cool,  and  the  water  will  be  twice  as  long  in 
parting  with  its  heat  as  the  oil.  Water,  therefore,  must 
receive  twice  as  much  heat  as  olive  oil  in  reaching  a  given 
temperature. 

The  relative  amount  of  heat  which  a  body  receives  in 
reaching  a  given  temperature  is  called  its  Specific  Heat,  or 
its  Capacity  for  Heat. 

549.  In  estimating  the  specific  heat  of  bodies,  that  of  water  is  taken  as  a 
standard.     Reckoning  the  specific  heat  of  water  as  1,  that*  of  iron  is  about 
]/9,  and  mercury  only  '/as-    As  a  general  thing,  the  densest  bodies  have  the 
ieast  specific  heat ;  solids  have  less  than  liquids,  and  liquids  less  than  gases 
and  vapors. 

550.  As  the  elastic  fluids  expand,  they  are  rarefied,  and  their  specific  heat 
becomes  greater. — that  is,  it  requires  more  heat  to  raise  them  to  a  given  tem- 
perature.    This  is  one  reason  why  the  upper  regions  of  the  atmosphere  are 
colder  than  the  lower,  as  is  found  by  those  who  ascend  mountains. 

Steam. 

551.  GENERATION  OF  STEAM. — "Water  is  rapidly  turned 
into  steam  at  its  boiling-point,  which  in  an  open  vessel  at 
the  level  of  the  sea  is  212°  F.  After  it  commences  boiling, 
water  can  not  be  raised  to  any  higher  temperature,  because 
all  the  heat  subsequently  applied  is  absorbed  by  the  steam 
and  passes  off  with  it. 

used  ?  Describe  the  Pyrometer.  548.  How  is  it  proved  that  water  must  receive  twice 
as  much  heat  as  olive  oil  in  reaching  a  given  temperature  ?  What  is  meant  by  Spe- 
cific Heat  ?  549.  In  estimating  the  specific  heat  of  bodies,  what  is  taken  as  a  stand- 
ard ?  What  is  the  specific  heat  of  iron  ?  Of  mercury  ?  Asa  general  thing,  what 
bodies  have  the  least  specific  heat?  550.  Under  what  circumstances  is  the  specific? 
heat  of  elastic  fluids  increased  ?  What  fact  is  thus  explained  ?  551.  How  is  steam 
generated  ?  Why  can  not  water,  after  it  commences  boiling,  be  raised  to  any  higher 


STEAM. 


217 


Fig.  219. 


If  the  water  is  in  a  close  vessel,  the  steam  first  formed, 
being  confined,  presses  on  the  water  and  prevents  it  from 
boiling  as  soon  as  before.  It  may  now  be  raised  to  a  more 
elevated  temperature,  for  heat  is  not  withdrawn  by  the 
formation  of  steam  till  it  reaches  a  higher  point. 

552.  Steam  has  the  same  temperature  as  the  water  from 
which  it  is  formed,  the  heat  absorbed  in  the  process  of  for- 
mation becoming  latent.     When  it  is  generated  from  wa- 
ter in  an  open  vessel,  its  temperature  is  212°;  in  a  confined 
vessel  it  will  be  higher,  according  to  the  pressure  on  the 
surface  of  the  water. 

553.  Steam  is  colorless  and  invisible.     When  cooled  by 
contact  with  the  atmosphere,  it  begins  to  turn  back  into  a 
liquid  state,  and  assumes  a  grey  mist-like  appearance.   Look 
at  the  spout  of  a  tea-kettle  full  of  boiling  water.     For  half 
an  inch  from  the  extremity  nothing  can  be  seen ;  beyond 
that,  the  steam,  cooling  and  beginning  to 

condense,  becomes  visible. 

554.  The  generation  and  properties  of  steam  may 
be  understood  from  Fig.  219.  AB  represents  the  in- 
side of  a  tall  glass  tube,  the  section  of  which  has  an 
area  of  one  square  inch.  The  tube  is  closed  at  its 
lower  end,  and  contains  a  cubic  inch  of  water,  D,  and 
resting  on  it  a  tightly-fitting  piston,  C.  A  cord,  fast- 
ened to  the  piston,  is  carried  round  the  wheel  E,  and 
attached  to  the  weight  F.  F  is  made  just  heavy  enough 
to  counterbalance  the  piston  and  its  friction  against 
the  tube.  Suppose  a  thermometer  to  be  placed  in 
the  water,  and  apply  heat  at  the  bottom  of  the  tube. 
As  soon  as  the  thermometer  indicates  a  temperature 
of  212°,  the  piston  begins  to  rise,  leaving  a  space  ap- 
parently empty  between  it  and  the  water.  The  fire 
continues  to  impart  heat  to  the  water,  but  the  mer- 
cury in  the  thermometer  remains  stationary  at  212° ; 
the  piston  keeps  rising,  and  the  water  begins  to  di- 
minish. If  the  process  were  continued  and  the  tube 
were  long  enough,  the  piston  would  at  last  reach  a 

temperature  ?  Under  what  circumstances  may  water  be  raised  to  a  higher  tempera- 
ture than  212°  ?  552.  What  is  the  temperature  of  steam  ?  553.  What  is  the  color  of 
etcam  ?  Explain  the  mist-like  appearance  a  short  distance  from  the  spout  of  a  boiling 
tea-kettle.  554.  With  the  aid  of  Fig.  219,  show  the  process  of  generating  steam,  and 

10 


218  PYBONOMICS. 

height  of  nearly  1,700  inches,  by  which  time  the  water  would  entirely  disap- 
pear. If  the  tube  were  then  weighed,  though  nothing  could  be  seen  in  it  but 
the  piston,  it  would  be  found  to  have  exactly  the  same  weight  as  at  first. 
The  water  would  simply  be  converted  into  steam,  and  thus  increased  in  vol- 
ume 1,700  times.  The  piston,  with  the  pressure  of  the  atmosphere  on  it 
(which  is  15  pounds,  the  area  of  the  piston  being  one  square  inch),  would  bs 
raised  1,700  inches. 

All  the  time  steam  is  forming,  a  uniform  amount  of  heat  is  applied  to  the 
tube.  As  the  mercury  in  the  thermometer  rises  no  higher  than  212°,  it  is 
evident  that  the  heat  imparted  after  it  reaches  that  point  is  absorbed  by  the 
steam  and  becomes  latent.  To  determine  the  amount  of  this  latent  heat,  we 
must  compare  the  time  required  to  raise  the  water  from  the  freezing  to  the 
boiling  point  with  the  time  that  elapses  from  the  commencement  of  boiling 
till  the  water  disappears.  We  shall  find  that  the  latter  interval  is  5Va  times 
as  great  as  the  former ;  and,  since  from  the  freezing-point  (32°)  to  the  boiling- 
point  (212°)  is  180°,  we  conclude  that  the  amount  of  heat  absorbed  is  5»/a 
times  180°,  or  nearly  1,000  degrees.  That  is,  the  heat  applied  would  have 
raised  the  water  to  a  temperature  of  nearly  1,000°,  if  it  could  have  remained 
in  the  liquid  state. 

555.  If,  besides  the  pressure  of  the  atmosphere  on  P,  a  weight  of  15  pounds 
were  placed  on  it,  it  would  be  said  to  have  a  pressure  of  two  atmospJieres. 
Steam,  in  this  case,  would  not  commence  forming  till  th'e  water  reached  a 
temperature  of  251V2  degrees ;  and,  when  the  whole  was  evaporated,  the  pis- 
ton would  stand  only  about  half  as  high  as  before.  Under  a  pressure  of  three 
atmospheres,  the  piston  would  be  raised  about  one-third  as  high,  &c. ;  the 
mechanical  force  developed  in  the  evaporation  of  a  given  quantity  of  water 
remaining  nearly  the  same.  This  force,  for  a  cubic  inch  of  water,  is  suffi- 
cient to  raise  a  ton  a  foot  high. 

556.  Steam  has  a  high  degree  of  elasticity  and  expansi- 
bility.   Under  a  pressure  of  two  atmospheres,  or  30  pounds 
to  the  square  inch,  it  would  raise  the  piston  in  the  above 
experiment  about  850  inches;  if  15  pounds  were  removed 
from  the  piston,  the  expansive  force  of  the  steam  would 
drive  it  up  850  inches  farther. 

557.  CONDENSATION  OF  STEAM. — Steam  retains  its  form 
only  as  long  as  it  retains  the  latent  heat  absorbed.     The 

describe  some  of  its  properties.  When  water  is  converted  into  steam,  how  many 
ftimes  is  its  volume  increased?  How  is  this  proved  with  the  apparatus  just  de- 
fcribed  ?  Prove  that  heat  becomes  latent  in  the  steam.  How  can  the  amount  of 
Aatent  heat  be  determined?  555.  When  is  steam  said  to  have  a  pressure  of  two  at- 
mospheres? How  high  would  the  piston  then  be  raised  ?  How  high  would  the  piston 
be  raised  under  a  pressure  of  three  atmospheres?  How  great  is  the  mechanical  force 
developed  in  evaporating  a  cubic  inch  of  water?  556.  Prove  the  expansibility  of 
(team.  557.  How  long  does  steam  retain  its  form  ?  When  is  it  condensed  ?  Show 


THE  STEAM-ENGINE.  219 

moment  it  is  forced  to  part  with  this  heat,  it  is  turned  back 
into  the  liquid  form,  or  condensed. 

In  the  above  experiment,  after  the  piston  has  been  raised  1,700  inches,  let 
the  fire  be  removed,  and  cold  water  be  applied  to  the  surface  of  the  tube. 
The  latent  heat  will  be  abstracted,  and  the  steam  will  be  condensed  and  form 
once  more  a  cubic  inch  of  water  at  the  bottom  of  the  tube.  As  the  steam 
•Condenses,  successive  vacuums  are  produced ;  and  the  piston,  forced  down 
by  the  pressure  of  the  atmosphere,  descends,  and  finally  rests  on  the  water 
as  at  first. 

By  applying  heat  again,  the  process  may  be  repeated.  An  up-and-down 
motion  may  in  this  way  be  communicated  to  the  piston ;  and  the  piston  may 
be  connected  with  machinery,  which  will  thus  be  set  in  motion  by  the  al- 
ternate evaporation  of  water  and  condensation  of  steam.  This  was  the  prin- 
ciple of  the  Atmospheric  Engine,  which  was  once  extensively  used,  but  has 
now  been  superseded. 

The  Steam-Engine. 

558.  HERO'S  ENGINE. — Steam  and  some  of  its  proper- 
ties appear  to  have  been  known  to  the  ancients  centuries 
before  the  Christian  era.  Hero,  of  Alexandria,  who  flour- 
ished about  200  years  B.  c.,  has  left  us  a  description  of  a 
steam-engine  by  which  machinery  could  be  set  in  motion. 

Fig.  220  represents  Hero's  Fig.  220. 

engine.  A  hollow  metallic 
globe  is  supported  by  pivots, 
and  provided  with  a  number 
of  jets  equally  distant  from 
the  pivots,  and  bent  at  right 
angles  near  their  outer  end. 
As  soon  as  steam  is  introduced 
into  the  globe,  it  issues  vio- 
lently from  the  mouth  of  each 
jet,  while  on  the  opposite  side 
of  each  it  presses  without  be- 
ing  able  to  escape.  This  un- 
balanced pressure  makes  the 
globe  revolve.  Machinery  may  HEEO'S  STEAM-ENGINE. 

be  set  in  motion  by  means  of  a  band  connected  with  this  apparatus. 

559.  Hero's  was  a  simple  rotatory  engine.    No  use  was  made  of  it  for 

how  it  may  be  condensed  in  the  above  experiment.  "What  follows  the  condensation 
of  the  steam  ?  How  may  an  up-and-down  motion  be  communicated  to  the  piston  ? 
What  engine  was  constructed  on  this  principle  ?  558.  How  long  ago  was  steam 
known  ?  Who  has  left  us  a  description  of  a  steam-engine  ?  Describe  Hero's  engine* 


220  PYRONOMICS. 

2,000  years ;  but  the  principle  involved  has  been  revived,  and  is  applied  ia 
rotatory  engines  at  the  present  day. 

560.  DE  GARAY'S  ENGINE. — In  1543,  a  Spaniard,  by  the 
name  of  De  Garay,  undertook  to  propel  a  vessel  of  200  tons 
in  the  harbor  of  Barcelona  by  the  force  of  steam.     He  kept 
his  machinery  a  secret,  but  it  was  observed  that  a  boiler 
and  two  wheels  constituted  the  principal  part  of  his  appa- 
ratus.    The  experiment  succeeded.      The  vessel  moved 
three  miles  an  hour,  and  was  turned  or  stopped  at  pleasure ; 
but  the  Emperor  Charles  V.,  by  whose  order  the  trial  was 
made,  never  followed  the  matter  up,  and  De  Garay  and  his 
invention  were  forgotten. 

561.  ENGINES  OP  DE  CATJS  AND  BRANCA. — In  1615,  De 
Caus,  a  French  mathematician,  devised  an  apparatus  by 
which  water  could  be  raised  in  a  tube  through  the  agency 
of  steam.     A  few  years  afterwards,  an  Italian  physician, 
named  Branca,  ground  his  drugs  by  means  of  a  wheel  set 
in  motion  by  steam.    The  steam  was  led  from  a  close  ves- 
sel, in  which  it  was  prepared,  and  discharged  against  flanges 
on  the  rim  of  the  wheel. 

562.  THE  MAKQUIS  OF  WORCESTER'S  ENGINE. — The  Mar- 
quis of  Worcester,  by  many  regarded  as  the  inventor  of  tho 
steam-engine,  greatly  improved  on  the  imperfect  attempts 
of  those  who  had  preceded  him. 

Some  say  that  Worcester  derived  his  ideas  from  De  Caus.  Others  claim 
that  his  invention  was  purely  original,  and  the  result  of  reflections  to  which 
he  was  led  during  his  imprisonment  in  the  Tower  of  London,  in  16."6,  for 
plotting  against  the  government  of  Cromwell.  Observing  how  the  steam  kept 
moving  the  lid  of  the  pot  in  which  he  was  cooking  his  dinner,  he  could  not 
help  thinking  that  this  power  could  be  turned  to  a  variety  of  useful  purposes, 
and  set  about  devising  an  engine  in  which  it  might  be  applied  to  the  raising 
of  water. 

The  Marquis  of  Worcester  generated  his  steam  in  a  boiler,  and  led  it  by 
pipes  to  two  vessels  communicating  on  one  side  with  the  reservoir  from 
which  it  was  to  be  drawn,  and  on  the  other  with  the  cistern  into  which  it 
was  to  be  discharged. 

659.  What  sort  of  an  engine  was  Hero's,  and  what  is  said  of  it  ?  560.  Give  an  account 
of  De  Garay's  engine,  and  the  experiment  made  with  it.  581.  Give  an  account  of  Do 
Caus's  engine.  Of  Branca's.  562.  Whom  do  many  regard  as  the  inventor  of  the  steam- 
engine  ?  "What  claim  has  he  to  the  honor  ?  How  was  he  led  to  reflect  on  the  subjoct  ? 


THE   STEAM-ENGINE. 


221 


563.  PAPIN'S  ENGINE. — The  next  step  was  taken  by  Pa- 
pin,  who  devised  the  mode  of  giving  a  piston  an  up-and- 
down  motion  in  a  cylinder  by  alternately  generating  and 
condensing  steam  below  a  piston. 

564.  SAVERY'S  ENGINE. — Captain  Thomas  Savery,  in 
1698,  constructed  an  engine  superior  to  any  before  invent- 
ed.    He  was  led  to  investigate  the  subject  by  the  following 
occurrence.    Having  finished  a  flask  of  wine  at  a  tavern,  he 
flung  it  on  the  fire,  and  called  for  a  basin  of  water  to  wash 
his  hands.     Some  of  the  wine  remained  in  the  flask,  and 
steam  soon  began  to  issue  from  it.     Observing  this,  Savery 
thought  that  he  would  try  the  eifect  of  inverting  the  flask 
and  plunging  its  mouth  into  the  basin  of  cold  water.     No 
sooner  had  he  done  this  than  the  steam  condensed,  and  the 
water  rushing  into  the  flask  nearly  filled  it.    Confident  that 
he  could  advantageously  apply  this  principle  in  machinery, 
Savery  rested  not  till  he  invented  an  engine  which  was  em- 
ployed with  success  in  drawing  off  the  water  from  mines. 


Fig.  221. 


565.  The  principle  on  which  Savery's  engine 
worked,  may  be  understood  from  Fig.  221.  S  is  a 
pipe  connecting  a  boiler  in  which  steam  is  genera- 
ted (and  which  does  not  appear  in  the  Figure)  with 
a  cylindrical  vessel,  C,  called  the  receiver.  I  is  known 
as  the  injection-pipe,  and  is  used  for  throwing  cold 
water  into  the  receiver  to  condense  the  steam.  The 
steam-pipe,  S,  and  the  injection-pipe,  I,  contain  the 
stop-cocks,  G,  B,  which  are  moved  by  the  common 
handle,  A,  so  arranged  that  when  one  is  opened  the 
other  is  closed.  F  is  a  pipe  which  descends  to  the 
reservoir  whence  the  water  is  to  be  drawn,  and  is 
commanded  by  the  valve  V,  opening  upward.  E  D 
is  a  pipe  leading  from  the  bottom  of  the  receiver  up 
to  the  cistern,  into  which  the  water  is  to  be  discharged.  This  pipe  contains 
the  valve  Q,  opening  upward. 

Operation. — To  work  the  engine,  open  the  stop-cock  G,  which  of  course 
involves  the  shutting  of  B.  The  steam  rushes  in  through  S,  and  fills  the  re- 
ceiver C,  driving  out  the  air  through  the  valve  Q.  When  C  is  full,  shut  G 


How  was  the  Marquis  of  Worcester's  apparatus  arranged  ?  563.  Who  took  the  next 
step  ?  What  was  Papin's  improvement  ?  564.  Who  constructed  a  superior  engine  in 
1698?  Eelate  the  circumstances  that  led  Savery  to  investigate  the  subject.  565.  With 
the  aid  of  Fig.  221,  describe  the  parts  of  Savery's  engine.  Explain  its  operation. 


222  PYRONOMICS. 

and  open  B.  Cold  water  at  once  enters  through  the  injection-pipe  and  con- 
denses the  steam  in  C.  A  vacuum  is  thus  formed,  and  the  water  in  the  res- 
ervoir or  mine,  under  the  pressure  of  the  atmosphere,  forces  open  the  valve 
V,  and  rushes  up  through  Finto  G,  till  the  receiver  is  nearly  filled.  G  is  then 
opened  and  B  closed ;  when  the  steam  again  enters  through  S,  and  by  its 
expansive  force  opens  the  valve  Q,  and  drives  the  water  up  through  E  D  into 
the  cistern. 

566.  NEWCOMEN'S  ENGINE. — Savery's  engine  was  era- 
ployed  only  for  raising  water ;  but  Newcomen,  an  intelli- 
gent blacksmith,  extended  its  sphere  of  usefulness,  by  con- 
necting a  piston,  worked  up  and  down  on  Papin's  principle, 
with  a  beam  turning  on  a  pivot,  by  means  of  which  ma- 
chinery of  different  kinds  could  be  set  in  motion. 

567.  About  this  time,  also,  the  engine  was  made  self-acting  through  the 
ingenuity  of  Humphrey  Potter,  a  lad  employed  to  turn  the  stop-cocks  Pre- 
ferring play  to  this  monotonous  labor,  he  contrived  to  fasten  cords  m  the 
beam  to  the  handle  of  the  stop-cocks,  in  such  a  way  that  the  latter  were 
opened  and  closed  at  the  proper  times,  while  he  was  away,  enjoying  himself 
with  his  companions.  His  device  was  after  a  time  found  out,  and  saved  so 
much  labor  that  it  was  at  once  adopted  as  an  essential  part  of  the  machine. 

568.  WATT'S  ENGINE. — The  genius  of  James  Watt 
brought  the  steam-engine  to  such  perfection  that  but  little 
improvement  has  since  been  made  in  it.  Gifted  with  re- 
markable mathematical  powers  and  a  reflective  mind,  he 
commenced  his  experiments  in  1763.  Having  been  em- 
ployed to  repair  one  of  Newcomen's  engines,  he  soon  per- 
ceived that  there  was  a  great  loss  in  consequence  of  having 
every  time  to  cool  down  the  receiver  from  a  high  degree 
of  heat  before  the  steam  could  be  condensed.  This  diffi- 
culty he  remedied  by  providing  a  separate  chamber  called 
a  condenser,  to  which  the  steam  was  conveyed  and  in  which 
\t  was  condensed.  He  also  made  the  movement  of  the  pis- 
ton more  prompt  and  effective  by  introducing  steam  into  the 
cylinder  alternately  above  and  below  it.  The  Double- 
acting  Condensing  Steam-engine,  as  improved  by  Watt,  and 

566.  What  was  the  only  purpose  for  which  Savery's  engine  was  employed  ?  Who  ex- 
tended its  usefulness,  and  how  ?  567.  Give  an  account  of  Humphrey  Potter's  im- 
provement, and  the  circumstances  under  which  it  was  devised.  56S.  Who  brought 
the  steam-engine  to  comparative  perfection  ?  When  did  Watt  commence  his  exper- 
iments? What  disadvantage  did  he  perceive  that  Newcomen's  engines  labored  un- 
der? How  did  he  remedy  the  difficulty  ?  What  other  improvement  did  he  make  ? 


THE   STEAM-ENGINE. 


223 


now  generally  constructed  for  manufacturing  establishments, 
is  represented  in  Fig.  222. 

569.  Description  of  the  Parts. — A  is  the  cylinder,  in  which  the  piston  T 
works.    This  piston  is  connected  by  the  piston-rod  R  with  the  working-beam 

Fig.  222. 


TUB   DOUBLE-ACTING   CONDENSING  STEAM-ENGINE. 

V  W,  which  turns  on  a  pivot,  U.  The  other  end  of  the  working-beam,  0, 
imparts  a  rotary  motion  to  the  heavy  fly-wheel  X  Y,  by  means  of  the  connect- 
ing-rod P  and  the  crank  Q.  The  fly,  as  explained  on  page  125,  regulates  the 
motion,  and  is  directly  connected  with  the  machinery  to  be  moved.  Steam 

669.  Describe  the  parts  of  Watt's  Double-acting  Condensing  Engine.    Show  how  th« 


224  PYKONOMICS. 

is  conveyed  to  the  cylinder  A  from  the  toiler  (which  is  not  seen  in  the  fig- 
ure), through  the  steam-pipe  B,  which  is  commanded  by  the  throttle-value  C. 
This  valve  is  connected  with  the  governor  D,  in  such  a  way  as  to  be  opened 
when  the  supply  of  steam  is  too  small  and  closed  when  it  is  too  great. 

Communicating  with  the  cylinder  at  its  top  and  bottom  on  the  left,  are 
two  hollo  w  steam-boxes,  E,  E,  each  of  which  is  divided  into  three  compartments 
by  two  valves.  F  is  called  the  upper  induction-valve,  and  opens  or  closes 
communication  between  the  steam-pipe  and  the  upper  part  of  the  cylinder, 
so  as  to  admit  or  intercept  a  supply  of  steam.  G,  called  the  upper  exhaustion- 
valve,  opens  or  closes  communication  between  the  upper  part  of  the  piston 
and  the  condenser  K,  so  that  the  steam  may  either  be  allowed  to  escape  into 
the  latter  or  confined  in  the  cylinder.  The  lower  induction-valve  g,  and  the 
lower  exhaustion-valve  f,  stand  in  the  same  relation  to  the  lower  part  of  the 
cylinder,  the  former  connecting  it  with  the  steam-pipe,  and  the  latter  with 
the  condenser  K.  These  valves  are  connected  by  a  system  of  levers  with  a 
common  handle,  H,  called  a  spanner,  which  is  made  to  work  at  the  proper  in- 
tervals by  a  pin  projecting  from  the  rod  L,  which  is  moved  by  the  working- 
beam.  The  spanner  works  so  as  to  open  and  close  the  valves  by  pairs.  When 
it  is  pressed  up,  it  opens  F  and/,  and  closes  G  and  g  ;  when  pressed  down, 
it  closes  F  and/  and  opens  G  and  g. 

Below  is  the  condensing  apparatus,  consisting  of  two  cylinders,  I  and  J, 
immersed  in  a  cistern  of  cold  water.  A  pipe,  K,  having  an  end  like  the  rose 
of  a  watering-pot,  conveys  water  from  the  cistern  to  the  cylinder  I  (the  sup- 
ply being  regulated  by  a  stop-cock),  and  thus  condenses  the  steam  which  is 
from  time  to  time  admitted  into  I.  The  other  cylinder,  J,  called  the  air-pump, 
contains  a  piston  with  a  valve  in  it  opening  upward,  which  works  like  the 
bucket  of  a  common  pump,  and  draws  off  the  surplus  water  that  collects  at 
the  bottom  of  the  cylinder  I  into  the  upper  reservoir  S.  The  hot-water  pump 
M  then  conveys  this  water  to  the  cistern  that  supplies  the  boiler.  To  keep 
the  water  around  the  condensing  apparatus  at  the  right  temperature,  a  fresh 
supply  is  constantly  introduced  through  the  cold-water  pump  N ;  which,  like 
the  hot- water  pump  and  the  air-pump,  is  kept  in  operation  by  rods  connected 
with  the  working-beam. 

570.  Operation.— The  working  of  the  engine  is  as  follows :— Let  the  piston 
be  at  the  top  of  the  cylinder,  and  all  the  space  below  be  filled  with  steam. 
The  upper  induction-valve  and  the  lower  exhaustion-valve  are  then  opened 
by  the  spanner,  while  the  upper  exhaustion-valve  and  the  lower  induction- 
valve  are  closed.  By  this  means  steam  is  introduced  above  the  piston,  while 
the  steam  beneath  is  drawn  off  into  the  condenser,  where  it  is  converted  into 
water.  The  pressure  of  the  steam  above  at  once  forces  the  piston  to  the  bot- 
tom of  the  cylinder.  Just  at  this  moment  the  spanner  is  moved  in  the  oppo- 
site direction,  and  the  valves  that  were  before  opened  are  closed,  while  those 
that  were  previously  closed  are  opened.  The  steam  is  now  admitted  beneath 
the  piston,  and  the  steam  above  is  drawn  off  into  the  condenser  and  convert- 
ed into  water  as  before.  While  this  action  is  going  on,  the  cold-water  pump 

valves  work.    Describe  the  condensing  apparatus.    570.  How  is  the  engine  worked? 


THE   STEAM-ENGINE. 


225 


is  constantly  supplying  the  cistern  in  which  the  condenser  is  immersed ;  while 
the  air-pump  is  drawing  off  the  hot  water  from  the  condenser  to  the  upper 
reservoir,  whence  it  is  conveyed  by  the  hot-water  pump  to  the  cistern  that 
supplies  the  boiler.  An  up-and-down  motion  is  thus  communicated  to  the 
piston,  and  by  it  to  the  working-beam,  which  causes  the  fly  to  revolve,  and 
moves  the  machinery  with  which  it  is  connected. 

571.  The  Governor. — The  Governor,  an  ingenious  piece 
of  mechanism,  by  which  the  throttle-valve  in  the  steam- 
pipe  is  opened  and  closed,  and  the  supply  of  steam  regu- 
lated as  the  machinery  requires,  is  worthy  of  further  de- 
scription. 

The  governor  and  its 
connection  with  the  throt- 
tle-valve are  represented  in 
Fig.  223.  It  consists  of  two 
heavy  balls  of  iron,  E,  E, 
suspended  by  metallic  arms 
from  the  point  e.  At  e  they 


THE   GOVERNOR. 


cross,  forming  a  joint,  and 
are  continued  to/,/  where 
they  are  attached  by  pivots 
to  other  bars,/ A,/  A.  These 
bars  are  joined  to  one  end 
of  a  lever,  the  other  end  of 
which,  H,  is  connected  at 
W  with  the  handle  of  the 
valve  Z. ,  The  spindle  D  D,  to  which  the  balls  are  attached,  turns  with  the 
fly-wheel.  When  the  fly-wheel  revolves  very  rapidly,  the  balls  E  E,  under 
the  influence  of  the  centrifugal  force,  fly  out  from  the  spindle,  and  with  the 
aid  of  the  bars/ A,/ A,  pull  down  the  end  of  the  lever  g.  The  other  end,  H, 
is  of  course  raised,  and  with  it  the  handle  of  the  valve  Z,  which  is  thus  made 
to  close  the  mouth  of  the  steam-pipe  A  and  cut  off  the  supply  of  steam.  On 
the  other  hand,  when  the  motion  of  the  fly  diminishes,  the  centrifugal  force 
of  the  balls  E  E  also  diminishes,  and  they  fall  towards  the  spindle.  The  near- 
er end  of  the  lever  g  is  thus  raised,  while  the  end  H  is  depressed.  The  valve 
Z  is  by  this  means  opened,  and  admits  a  full  supply  of  steam.  The  governor 
thus  acts  almost  with  human  intelligence,  now  admitting,  and  now  cutting 
off  the  steam,  just  as  is  required. 

572.  The  Boiler. — The  boiler  is  made  of  thick  wrought- 
irou  or  copper  plates,  riveted  as  strongly  as  possible,  so  as 
to  resist  the  expansive  force  of  the  steam  generated  within. 

How  are  the  cisterns  supplied  ?    571.  What  is  the  Governor  ?    Describe  the  gov- 
ernor, and  its  connection  with  the  throttle -valve.    Bhow  the  workings  of  the  gov- 

10* 


226  PYKONOMICS. 

The  fire  is  applied  in  an  apartment  beneath  or  within  the 
boiler  called  the  Furnace. 

Boilers  are  made  of  different  shapes,  but  are  generally 
cylindrical,  because  this  form  is  one  of  the  strongest.  Watt 
made  his  concave  on  the  bottom,  in  order  to  bring  a  greater 
extent  of  surface  in  contact  with  the  flame. 

573.  The  Safety  Valve. — The  pressure  on  the  boiler,  in 
consequence  of  the  expansive  force  of  steam,  is  immense. 
If  it  is  allowed  to  become  too  great,  the  boiler  bursts,  often 
with  fatal  effects.     To  prevent  such  catastrophes,  a  Safety 
Valve  is  fixed  in  the  upper  part  of  the  boiler,  which  is  forced 
open  and  allows  some  of  the  steam  to  escape  whenever  the 
pressure  exceeds  a  certain  amount.     A  lever,  with  a  weight 
which  slides  to  and  fro  on  its  arm,  is  attached  to  the  valve  ; 
and  the  engineer,  by  placing  the  weight  at  different  dis- 
tances, can  determine  the  amou/it  of  pressure  which  the 
boiler  shall  sustain  before  the  valve  will  open. 

574.  KINDS  OF  ENGINES. — Engines  are  divided  into  two 
kinds,  Low  Pressure  and  High  Pressure. 

In  the  Low  Pressure  Engine,  one  form  of  which  has  been 
described  above,  the  steam  is  carried  off  and  condensed ; 
while  in  the  High  Pressure  Engine  it  is  allowed  to  escape 
into  a  chimney,  and  thence  into  the  open  air.  The  latter, 
having  no  condensing  apparatus,  is  much  the  simpler  in  its 
construction.  It  is  noisy  when  in  operation,  in  consequence 
of  the  puffing  sound  made  by  the  steam* as  it  escapes. 

575.  As  regards  their  use,  engines  may  be  divided  into 
three  classes ;  Stationary  Engines,  employed  in  manufactur- 
ing, Marine  Engines,  for  propelling  boats,  and  Locomotive 
Engines,  for  drawing  wheeled  carriages. 

576.  THE  LOCOMOTIVE  ENGINE. — The  Locomotive  is  a 
high  pressure  engine.    The  principle  on  which  it  works  may 
be  understood  from  Fig.  224. 

•rnor.  572.  Of  what  is  the  boiler  made?  Where  is  the  fire  applied?  What  is  the 
usual  shape  of  boilers  ?  What  shape  did  Watt  make  his,  and  why  ?  573.  What  is  the 
use  of  the  Safety  Valve  ?  How  is  it  worked  ?  574.  How  are  engines  divided  ?  What 
constitutes  the  difference  between  Low  Pressure  and  High  Pressure  Engines  ?  Which 
lire  the  simpler?  Which  are  the  more  noisy,  and  why?  575.  As  regards  their  use, 


THE  LOCOMOTIVE  ENGINE. 

Fig.  224 


227 


The  cylinder  A  in  this  engine  is  horizontal  instead  of  vertical,  and  the  pis- 
ton works  horizontally.  B,  the  piston-rod,  is  connected  by  a  crank,  D,  with 
the  axle  E  E  of  the  wheels,  F,  F.  The  piston,  moving  alternately  in  and  out 
of  the  cylinder,  with  the  aid  of  the  crank  causes  the  axle  and  wheels  to  re- 
volve ;  and  the  wheels,  by  their  friction  on  the  rails,  move  forward  the  en- 
gine and  whatever  may  be  attached  to  it.  The  heavy  line  represents  the 
position  of  the  parts  when  the  piston  is  at  the  remote  extremity  of  the  cylin- 
der; the  dotted  line  shows  their  position,  when  the  piston  has  reached  the 
other  end.  Steam  is  first  introduced  on  one  side  of  the  piston,  and  then  on 
the  other,  being  allowed  to  escape  as  soon  as  it  has  done  its  work, — that  is, 
driven  the  piston  to  the  opposite  extremity.  The  rest  of  the  machinery  con- 
sists of  arrangements  for  boiling  the  water,  for  regulating  the  admission  of 
steam  into  the  cylinder  and  its  discharge,  for  providing  draught  for  the  fire, 
and  for  giving  the  driver  the  means  of  starting  and  stopping  the  engine,  and 
reversing  the  direction  of  its  motion. 

577.  History. — Watt  seems  to  have  been  the  first  to 
conceive  the  idea  of  propelling  wheeled  carriages  by  steam ; 
but  he  was  so  engaged  in  perfecting  the  stationary  engine 
that  he  did  not  attempt  to  carry  out  his  idea.  William 
Murdoch,  in  1784,  first  constructed  a  locomotive.  Though 
little  more  than  a  toy,  it  worked  successfully,  and  travelled 
so  fast  that  on  one  occasion  its  inventor  in  vain  tried  to 
keep  pace  with  it. 

Eighteen  years  passed  before  any  use  was  made  of  Mur- 
doch's invention  ;  at  the  end  of  that  time,  in  1802,  Richard 
Trevithick  publicly  exhibited  a  locomotive  engine,  so  con- 


into  what  three  classes  may  engines  be  divided  ?  576.  With  Fig.  224,  show  the  prin- 
ciple on  which  the  locomotive  engine  works.  What  does  the  rest  of  the  machinery 
consist  of?  577.  Who  first  conceived  the  idea  of  the  locomotive  engine  ?  Who  first 
carried  out  the  idea  ?  What  is  said  of  Murdoch's  engine  ?  Who  exhibited  an  im- 


228  EXAMPLES   FOE   PRACTICE. 

structed  that  it  could  be  used  for  transporting  cars.  Im- 
portant modifications  and  improvements  have  since  been 
made,  for  many  of  which  the  world  is  indebted  to  George 
Stephenson,  who  shares  with  Trevithick  the  honor  of  this 
great  invention. 

EXAMPLES  FOR  PRACTICE. 

1.  (See  §  510.)  A  joint  of  meat  stands  2  feet  from  a  fire,  a  fowl  4  feet;  how 

does  the  heat  which  strikes  the  former  compare  with  that  received  by 
the  latter  ? 

2.  How  does  the  heat  which  my  finger  receives  from  the  blaze  of  a  candle, 

when  held  an  inch  from  it,  compare  with  what  it  receives  when  held  a 
foot  from  it  ? 

3.  If  we  were  but  one-fifth  of  our  present  distance  from  the  sun,  how  many 

times  as  much  heat  would  we  receive  from  it  ? 

4.  The  planet  Neptune  is  about  30  times  as  far  from  the  sun  as  the  earth  is ; 

how  does  its  solar  heat  compare  with  ours  ? 

5.  To  receive  a  certain  amount  of  heat  from  a  fire,  an  object  is  placed  3  feet 

from  it ;  to  receive  only  one-fourth  as  much  heat,  how  far  from  the  fire 
must  it  be  placed  ? 

G.  (See  §526.)  A  quantity  of  water  at  the  freezing-point  measures  22  gallons  ; 
how  much  will  it  measure  when  its  temperature  has  increased  to  the 
boiling-point  ? 

7.  I  have  a  vessel  which  holds  46  gallons ;  how  much  water  at  a  temperature 

of  32°  must  I  put  in  it,  to  exactly  fill  the  vessel  when  it  boils  ? 

8.  "What  will  be  the  increase  in  measure  of  18  gallons  of  alcohol,  when  raised 

from  32°  to  212°  ?    What  will  be  the  increase  in  weight  ? 

9.  (See  §  554.)  Under  a  pressure  of  one  atmosphere,  how  many  cubic  inches 

of  steam  will  be  generated  from  2  cubic  inches  of  water?    From  10  cubic 
inches  of  water  ? 

10.  If  3,400  cubic  feet  of  steam  (under  a  pressure  of  one  atmosphere)  be  con- 
densed, how  much  water  will  it  make  ? 

11.  (See  §  555.)  Under  a  pressure  of  two  atmospheres,  about  how  many  cubic 
inches  of  steam  will  two  inches  of  water  generate  ?    How  many,  under 
a  pressure  of  three  atmospheres  ? 

12.  About  how  many  cubic  inches  of  steam  will  be  required,  to  raise  10  tons 
10  feet  high  ?    If  the  steam  were  condensed,  how  many  cubic  inches  of 
water  would  it  make  ? 

proved  locomotive  in  1802  ?  Who  subsequently  made  important  improvements  in 
the  locomotive? 


OPTICS.  229 


CHAPTER  XIV. 

OPTICS. 

578.  OPTICS  is  the  science  that  treats  of  light  and  vision. 

Nature  of  Light. 

579.  Light  is  an  agent,  by  the  action  of  which  upon  the 
eye  we  are  enabled  to  see. 

Light  is  imponderable ;  for  it  moves  with  great  velocity,  and  if  it  had 
any  weight,  though  it  were  ever  so  little,  its  striking  force  would  be  felt  by 
every  object  with  which  it  comes  in  contact.  Yet  it  does  not  affect  even  the 
most  sensitive  balance. 

580.  With  respect  to  the  nature  of  light,  two  theories 
have  been  advanced,  the  Corpuscular  and  the  Undulatory. 

581.  Corpuscular  Theory. — The  Corpuscular  Theory  teaches  that  light 
consists  of  extremely  minute  particles  of  matter,  thrown  off  from  luminous 
bodies,  which  strike  the  eye  and  produce  the  sensation  of  light,  just  as  par- 
ticles thrown  off  by  an  odoriferous  substance  affect  the  organ  of  smell.     This 
theory,  held  as  long  ago  as  the  days  of  Pythagoras,  was  received  by  New- 
ton ;  but,  failing  to  account  for  many  of  the  facts  more  recently  discovered 
iu  connection  with  light,  it  has  now  but  few  supporters. 

582.  Undulatory  Theory. — According  to  the  Undulatory  Theory,  light  is 
produced  by  the  undulations  of  an  exceedingly  subtile  imponderable  medi- 
um, known  as  Ether,  with  which  space  is  filled ;  just  as  sound  is  produced  by 
the  vibrations  of  air.    A  luminous  object  millions  of  miles  away  causes  the 
ether  in  contact  with  it  to  move  in  minute  waves,  like  the  surface  of  a  pond 
rippled  by  throwing  in  a  stone.     These  undulations  are  transmitted  with  in- 
conceivable rapidity,  till  they  reach  the  eye,  strike  the  sensitive  membrane 
that  lines  it,  and  produce  the  phenomena  of  vision.     This  theory,  advanced 
by  Descartes  [da-isar?],  but  first  definitely  laid  down  by  Huygens,  explains 
most  of  the  phenomena  of  optics,  and  is  now  generally  received. 

578.  What  is  Optics  ?  579.  What  is  Light  ?  How  is  it  proved  that  light  is  impon- 
derable ?  5SO.  What  two  theories  have  been  advanced  with  respect  to  the  nature  ol 
light  ?  581.  State  the  chief  points  of  the  Corpuscular  Theory.  By  whom  was  it  held  ? 
582.  According  to  the  Undulatory  Theory,  how  is  light  produced  ?  By  whom  was  tho 
Undulatory  Theory  advanced  ?  Which  of  these  theories  is  now  generally  received  ! 


230  OPTICS. 

583.  Rays. — Rays  are  single  lines  of  light,  the  smallest 
distinct  parts  into  which  light  can  be  resolved. 

Fig.  225.  rig.  226.  Fig.  227.  R  ay s  of  light  from  the 

same  body  either  move  in 
parallel  lines,  as  in  Fig. 
225  ;  or  diverge,  that  is,  sep- 
arate from  each  other,  as  in 
Fig.  226 ;  or  converge,  that 
is,  come  together  at  a  point  called  the  Focus,  as  in  Fig.  227. 

A  Beam  of  light  is  a  collection  of  parallel  rays. 

A  Pencil  of  light  is  a  collection  of  rays  not  parallel. 

A  Diverging  Pencil  is  a  collection  of  diverging  rays. 

A  Converging  Pencil  is  a  collection  of  converging  rays. 

Division  of  Bodies. 

584.  SELF-LUMINOUS  AND  NON-LUMINOUS  'BODIES. — As 
regards  the  production  of  light,  bodies  are  divided  into  two 
classes,  Self-luminous  and  Non-luminous. 

Self-luminous  bodies  are  those  which  are  seen  by  the 
light  that  they  themselves  produce  ;  as,  the  sun,  the  stars, 
a  lighted  candle. 

Non-luminous  bodies  are  those  that  produce  no  light  of 
their  own,  but  are  seen  only  by  that  of  other  bodies.  The 
moon  is  non-luminous,  its  light  being  borrowed  from  the 
sun.  The  furniture  in  a  dark  room  is  non-luminous,  being 
invisible  until  the  light  of  the  sun,  a  lamp,  or  some  other 
luminous  body,  is  admitted. 

Many  non-luminous  bodies,  when  exposed  to  a  heat  of  977°  F.,  become 
incandescent,  and  grow  brighter  and  brighter  with  every  increase  of  temper- 
ature beyond  that  point,  till  they  reach  a  white  heat.  This  is  a  striking  proof 
of  the  connection  between  light  and  heat. 

585.  TRANSPARENT,  TRANSLUCENT,  AND  OPAQUE  BODIES. 


583.  What  are  Ea^s  ?  How  may  rays  move  ?  What  is  a  Beam  of  light?  What  is  a  Pen- 
cil of  light  ?  What  is  a  Diverging  Pencil  ?  What  is  a  Converging  Pencil  ?  584.  As 
regards  the  production  of  light,  how  are  bodies  divided  ?  What  are  Self-luminous 
bodies?  What  are  Non-luminous  bodies?  Give  examples.  What  striking  proof 
kave  we  of  the  connection  between  light  and  heat  ?  585.  As  regards  the  transmission 


TRANSPARENT   AND    OPAQUE  BODIES.  231 

—As  regards  the  transmission  of  light,  bodies  are  divided 
into  three  classes  ;  Transparent,  Translucent,  and  Opaque. 

Transparent  bodies  are  such  as  allow  light  to  pass  freely 
through  them  ;  air,  water,  glass,  are  transparent. 

Translucent  bodies  are  such  as  allow  light  to  pass  through 
them,  but  not  freely ;  ground  glass,  thin  horn,  paper,  are 
translucent. 

Opaque  bodies  are  such  as  do  not  allow  light  to  pass 
through  them  ;  wood,  stone,  the  metals,  are  opaque. 

Transparent  and  opaque  are  relative  terms.  No  substance  transmits 
light  without  intercepting  some  by  the  way.  It  is  computed  that  the  sun's 
rays  lose  nearly  one-fourth  of  their  brilliancy  by  passing  through  the  earth's 
atmosphere  ;  and  that,  if  this  atmosphere  extended  fifteen  times  as  far  from 
the  surface  as  it  now  does,  we  should  receive  no  light  at  all  from  the  sun, 
but  should  be  plunged  in  perpetual  night.  On  the  other  hand,  an  opaque 
substance,  if  made  very  thin,  may  become  transparent.  Gold  leaf,  for  in- 
stance, held  in  the  sun's  rays,  transmits  a  dull  greenish  light. 

586.  MEDIA. — By  a  Medium  (plural,  media)  is  meant 
any  substance  through  which  a  body  or  agent  moves  in 
passing  from  one  point  to  another.     Air  is  the  medium  in 
which  birds  fly ;  water,  the  medium  in  which  fish  swim ; 
ether,  the  medium  in  which  the  planets  move.     In  connec- 
tion with  light,  any  substance  through  which  it  passes  is  a 
medium  ;  as  air,  water,  glass,  &c. 

587.  A  Uniform  Medium  is  one  that  is  of  the  same 
composition  and  density  throughout. 

Sources  of  Light. 

588.  The  principal  sources  of  light  are  nearly  the  same 
as  those  of  heat ;  viz.,  the  Sun  and  Stars,  Chemical  Action, 
Mechanical  Action,  Electricity,  and  Phosphorescence. 

Most  of  our  artificial  light  is  produced  by  chemical  action,  as  exhibited  in 
the  process  of  combustion  (see  §  479).  To  this  is  due  the  light  of  lamps,  can- 

of  light,  how  are  bodies  divided?  What  are  Transparent  bodies?  What  are  Trans- 
lucent bodies  ?  What  are  Opaque  bodies  ?  What  is  said  of  the  terms  transparent 
and  opaque  f  How  much  of  their  brilliancy  do  the  sun's  rays  lose  in  passing  through 
the  atmosphere  ?  What  would  be  the  consequence  if  the  atmosphere  extended  fif- 
teen times  as  far  as  at  present  ?  How  may  an  opaque  substance  be  made  transparent? 
586.  What  is  a  Medium?  Give  examples.  587.  What  is  a  Uniform  Medium? 
5SS.  Name  the  principal  sources  of  light.  How  is  most  of  our  artificial  light  pro- 


232  OPTICS. 

dies,  gas,  fires,  &c. — The  mechanical  action  involved  in  percussion  is  also  » 
source  of  light.  Sparks  are  produced  when  flint  and  steel  are  struck  vio- 
lently together. — Lightning  and  the  sparks  given  off  from  the  electrical  ma- 
chine are  examples  of  light  produced  by  electricity. — Phosphorescent  light  is 
unaccompanied  with  heat.  It  is  seen  in  decayed  wood,  fire-flies,  glow-worms, 
and  certain  marine  animals.  Vast  tracts  of  ocean  are  sometimes  rendered 
luminous  by  myriads  of  phosphorescent  creatures. 

589.  THE  SUN  AND  STARS,  SOURCES  OP  LIGHT. — The  sun 
lias  already  been  mentioned  (§  474)  as  the  great  natural 
source  of  heat  and  light  to  the  earth.  Notwithstanding 
the  loss  of  some  of  its  brightness  in  consequence  of  passing 
through  our  atmosphere,  its  light  is  more  intense  than  any 
other  with  which  we  are  acquainted.  The  most  dazzling 
artificial  lights  look  like  black  specks,  when  held  up  be- 
tween the  eye  and  the  sun,  so  much  more  brilliant  is  the 
latter.  It  would  require  the  concentrated  brightness  of 
5,563  wax  candles  at  the  distance  of  a  foot,  to  equal  the 
light  which  we  receive  from  the  sun  at  a  distance  of 
95,000,000  miles. 

The  fixed  stars  are  the  suns  of  other  systems.  Like  our 
sun,  they  are  self-luminous,  and  therefore  sources  of  light, 
though  unimportant  to  us  as  such  by  reason  of  their  great 
distance.  The  light  we  get  from  Sirius,  one  of  the  bright- 
est of  the  fixed  stars,  is  only  «ne  twenty-thousand-millionth 
of  what  we  receive  from  the  sun.  When  the  sun  shines, 
the  stars  are  invisible,  their  light  being  lost  in  his  superior 
brightness. 

The  light  of  some  of  the  stars  is  so  faint,  that  it  is  entirely  absorbed  by 
the  atmosphere  before  it  reaches  the  eye  of  an  observer  at  the  level  of  the  sea. 
This  is  the  reason  why  more  stars  are  visible  from  the  top  of  a  mountain  than 
from  its  base. 

590.  The  moon  and  planets  are  non-luminous,  receiving  from  the  sun  the 


duced  ?  Give  an  example  of  light  produced  by  mechanical  action.  Of  light  pro- 
duced by  electricity.  What  is  the  peculiarity  of  phosphorescent  light  ?  In  what  is 
it  seen  ?  5S9.  What  is  the  great  natural  source  of  light  to  the  earth  ?  How  does  the 
sun's  light  compare  with  other  lights  with  which  we  are  acquainted  ?  Prove  this. 
To  how  many  wax  candles  is  the  light  received  from  the  sun  equal  ?  What  are  tha 
fixed  stars  ?  What  renders  them  unimportant  to  us,  as  sources  of  light  ?  How  does 
the  light  of  Sirius  compare  with  that  of  the  sun?  Why  are  the  stars  invisible  in  tha 
day-time  ?  Why  can  more  stars  be  seen  from  the  top  of  a  mountain  than  from  its 
base  ?  590.  What  heavenly  bodies  are  non-luminous  ?  What  follows  with  respect  to 


PROPAGATION   OP  LIGHT.  233 

light  with  which  they  shine.  This  light,  reflected  to  the  earth,  is  much  inferior 
iu  brightness  to  that  received  directly  from  the  sun.  The  latter  body,  for 
example,  gives  us  800,000  times  as  much  light  as  the  moon. 

Propagation  of  Light. 

591.  DIRECTION. — Light  radiates  from  every  point  of  a 
luminous  surface  in  every  direction. 

The  flame  of  a  can-die  can  be  seen  by  thousands  of  persons  at  once,  be- 
cause a  ray  from  the  flame  meets  the  eye  of  each.  Within  the  immense  space 
belonging  to  the  solar  system,  there  is  no  point  at  which  an  observer  can  be 
placed  without  seeing  the  sun,  provided  no  opaque  body  intervenes.  From 
the  sun,  therefore,  and  from  every  luminous  body,  an  infinite  number  of  rays 
proceed. 

592.  In  a  uniform  medium,  light  is  propagated  in 
straight  lines. 

Look  through  a  straight  tube  at  the  sun,  and  you  see  it ;  not  so,  if  you 
look  through  a  bent  or  curved  tube.  Place  a  book  between  your  eye  and  a 
gas-burner;  the  latter  is  not  visible,  because,  to  reach  your  eye,  the  light  from 
it  would  have  to  deviate  from  a  straight  line.  Darken  a  room,  and  admit  a 
sunbeam  through  a  small  hole  in  a  shutter.  Its  path,  marked  out  by  the 
floating  dust  that  it  illuminates,  is  seen  to  be  a  straight  line. 

593.  The  rays  proceeding  in  straight  lines  from  different  particles  of  a 
luminous  body  cross  at  every  point  within  the  sphere  of  its  illumination,  but 
without  at  all  interfering  with  each  other ;  just  as  different  forces  may  act 
on  an  object,  and  each  produce  the  same  effect  as  if  it  acted  alone.  A  dozen 
candles  will  shine  through  a  hole  in  the  wall  of  a  dark  room,  and  each  with 
the  same  intensity  and  direction  as  if  no  other  rays  than  its  own  traversed 
the  narrow  passage. 

594.  VELOCITY. — Light  travels  with  the  enormous  ve- 
locity of  192,000  miles  in  a  second.  While  you  count  one, 
it  goes  eight  times  round  the  earth ;  it  would  take  the  swift- 
est bird  three  weeks  to  fly  once  around  it.  Light  traverses 
the  space  between  the  sun  and  the  earth  in  about  8  min- 
utes ;  a  cannon-ball  would  be  seventeen  years  in  going  the 
same  distance. 


their  light?  How  does  the  moon's  light  compare  with  the  sun's  ?  591.  What  is  th<» 
law  for  the  direction  of  radiated  light  ?  Show  the  truth  of  this  law  in  the  case  of  » 
candle  and  the  sun.  592.  In  a  uniform  medinrn,  how  is  light  propagated  ?  Prove 
this  by  some  familiar  experiments.  593.  What  is  said  of  the  rays  proceeding  in 
Btraight  lines  from  different  particles  of  a  luminous  body  ?  Illustrate  this  with  can- 
dles shining  through  a  hole.  594  What  is  the  velocity  of  light  ?  How  does  it  com- 
pare with  that  of  the  swiftest  bird  ?  With  that  of  a  cannon-ball  T  By  whom  was  the 


234  OPTICS. 

The  -velocity  of  light  was  discovered  accidentally,  by  Roemer,  an  eminent 
Danish  astronomer,  when  engaged  in  a  series  of  observations  on  one  of  the 
moons  of  the  planet  Jupiter.  This  moon,  in  a  certain  part  of  its  path,  be- 
comes invisible  to  an  observer  on  the  earth,  in  consequence  of  getting  be- 
hind its  planet.  Knowing  that  the  revolutions  of  the  moon  must  be  per- 
formed in  the  same  time,  Roemer  supposed  that  the  intervals  between  these 
invisible  periods  would  of  course  be  uniform.  To  his  surprise,  he  found  that 
they  differed  a  little  every  time ;  increasing  for  six  months  (at  the  expiration 
of  which,  the  eclipse  was  sixteen  minutes  later  than  at  first),  and  then  de-J 
creasing  at  the  same  rate  for  a  similar  period,  till  at  the  end  of  a  year  he 
found  the  interval  precisely  the  same  as  at  first.  The  conclusion  was  inevi- 
table. The  discrepancy  was  caused  by  the  difference  in  the  earth's  distance. 
If  the  first  observation  was  made  when  the  earth  was  at  that  point  of  her 
orbit  which  was  nearest  to  Jupiter,  six  months  afterwards  she  would  be  at 
the  most  distant  point;  and  the  light  from  Jupiter's  moon,  to  reach  the  ob- 
server's eye,  would  have  to  travel  the  whole  distance  across  the  orbit  (about 
190,000,000  miles)  farther  than  before.  Here  was  the  key  to  a  grand  discov- 
ery. If  light  was  sixteen  minutes,  or  960  seconds,  in  travelling  190,000,000 
miles,  it  was  easy  to  find  how  far  it  travelled  in  one  second. 

595.  INTENSITY  AT  DIFFERENT  DISTANCES. — The  intensi- 
ty of  light  diminishes  according  to  the  square  of  the  dis- 
tance from  the  luminous  body  that  produces  it. 

Let  several  objects  be  placed  respectively  1  foot,  2  feet,  3  feet,  &c.,  from  a 
luminous  body ;  they  will  then  receive  different  degrees  of  light  proportioned 
to  each  other  as  1,  »/4,  1/9t  &c. — A  planet  twice  as  far  from  the  sun  as  the 
earth  is,  would  receive  from  it  only  J/4  as  much  light  j  one  three  times  as  far, 
»/»  as  much ;  one  ten  times  as  far,  Vioo  as  much. 

Fi<T  228  596.  This  is  illustrated  with  Fig.  228.    A 

square  card  placed  at  A,  a  distance  of  1  foot 
from  the  candle,  receives  from  a  given  point  in 
the  flame  a  certain  amount  of  light.  This  same 
light,  if  not  intercepted  at  A,  goes  on  to  B  at  a 
distance  of  2  feet ;  it  there  illuminates  four 
squares  of  the  same  size  as  the  card,  and  has, 
therefore,  but  one-fourth  of  its  former  intensity. 
If  allowed  to  proceed  to  C,  3  feet,  it  illuminates  nine  such  squares,  and  has 
but  one-ninth  of  its  original  intensity,  &c. 

Shadows. 

597.  Light  falling  on  an  opaque  body  is  intercepted. 

velocity  of  light  discovered  ?  State  the  facts  and  reasoning  by  which  Koeraer  arrived 
at  this  discovery.  595.  What  is  the  law  relating  to  the  intensity  of  light  at  different 
distances?  Give  examples.  596.  Illustrate  this  law  with  Fig.  228.  597.  "What  la 


SHADOWS. 

The  darkness  thus  produced  behind  the  op 
called  its  Shadow. 

598.  Shadows  are  not  all  equally  dark.  They  may  be  more  or  l£§a  illu- 
mined by  reflected  light  or  by  rays  from  some  luminous  body  that  a£«[/iot 
intercepted.  Thus,  if  there  are  two  lighted  candles  in  different  part#^>£ »  -. 

room,  the  shadow  cast  by  either  is  less  dark  than  if  it  were  burning  alofi<e/*          \jK 
Again,  the  brighter  the  light  that  produces  a  shadow,  the  darker  it  appeanKjf 
by  contrast.     Hence,  to  compare  the  intensity  of  different  lights,  observe  the  • 
•hadows  respectively  cast  at  equal  distances ;  the  one  that  throws  the  dark**^-,.^^^ 
«st  shadow  is  the  brightest  light. 

599.  When  the  luminous  body  is  larger  than  the  opaque 
body  it  shines  on,  the  latter  throws  a  shadow  smaller,  than 
itself;  and  this  shadow  diminishes  according  to  the  dis- 
tance of  the  surface  on  which  it  is  thrown. 

In  Fig.  229,  let  A  be  a  luminous,  and  Fig.  229. 

B  an  opaque,  body.  B's  shadow,  no  mat- 
ter how  near  the  surface  on  which  it  is 
thrown,  must  be  smaller  than  B  itself; 
and,  as  the  surface  is  removed  from  B,  the 
shadow  diminishes,  till  it  is  reduced  to  a  point  at  C. 

If,  on  the  contrary,  the  opaque  body  is  the  larger  of  the 
two,  it  throws  a  shadow  greater  than  itself;  and  this  shad- 
ow increases  according  to  the  distance  of  the  surface  on 
which  it  is  thrown. 

600.  THE  PENUMBRA. — Every  luminous  body  has  an  in- 
finite number  of  points,  from  each  of  which  proceeds  a  pen- 
cil of  rays.     When  an  opaque  body  is  interposed,  some  of 
the  space  behind  it  is  cut  off  from  all  the  rays  of  the  lumi- 
nous body,  and  this  constitutes  the  shadow  proper.     Part 


of  the  space,  however,  while  it 
is  cut  off  from  some  of  the  rays, 
is  illumined  by  others;  this  is 
called  the  Penumbra. 

In  Fig.  230,  let  0  P  be  the  flame  of  a 
candle,  and  AB  an  opaque  object  placed  be- 
fore it.  The  space  A  B  C  D  is  not  reached 


Fig.  230. 


SHADOW   AND  PENUMBRA. 


meant  by  a  body's  Shadow  ?  598.  Why  are  not  all  shadows  equally  dark  ?  How 
may  we  compare  the  intensity  of  different  lights?  599.  When  does  a  body  throw  a 
•hadow  smaller  than  itself?  Illustrate  this  law  with  Fig.  229.  When  does  a  body 
throw  a  shadow  larger  than  itself?  600.  What  is  meant  by  the  Penumbra?  How  is 


230  OPTICS. 

by  any  ray  from  0  P,  and  is  therefore  the  Shadow  of  A  B.  The  space  A  E  C, 
while  it  is  cut  off  from  the  rays  produced  by  the  lower  extremity  of  the  flame, 
is  illumined  by  its  upper  extremity ;  hence  it  is  nowhere  so  dark  as  the  shad- 
ow, and  becomes  lighter  and  lighter  as  the  line  AE  is  approached.  So  the  space 
B  D  F  is  cut  off  from  the  rays  produced  by  the  upper  part  of  the  flame,  but 
receives  those  from  the  lower  part,  and  is  therefore  partially  illuminated. 
The  spaces  ACE,  B  D  F,  constitute  the  Penumbra,  or  imperfect  shadow, 
ofAB. 

Reflection  of  Ligbt. 

601.  When  light  strikes  an  opaque  body,  some  of  it  is 
absorbed,  and  some  reflected,  or  thrown  back  into  the  me- 
dium from  which  it  came.     According  to  the  Undulatory 
Theory,  we  should  say  that  some  of  the  undulations  that 
strike  the  opaque  body  are  brought  to  rest,  while  others 
are  reproduced  hi  the  same  medium  with  a  different  direc- 
tion from  what  they  had  before. 

The  reflection  of  light  is  analogous  to  the  reflected  motion  of  an  india 
rubber  ball  thrown  against  a  solid  surface.  It  is  by  the  light  irregularly  re- 
flected from  their  surfaces  that  all  non-luminous  bodies  are  seen. 

Transparent  surfaces,  as  well  as  opaque,  reflect  some  of  the  light  that 
strikes  them ;  otherwise,  they  would  not  be  visible.  We  see  overhanging 
objects  mirrored  in  a  stream  with  great  distinctness,  because  a  portion  of  the 
rays  received  from  them  are  reflected  by  the  water  to  our  eyes. 

602.  That  branch  of  Optics  which  treats  of  the  laws 
and  principles  of  reflected  light,  is  called  CATOPTRICS. 

603.  Rays  that  strike  a  body  are  called  Incident  Rays. 

604.  REFLECTIVE  POWER   OF  DIFFERENT  SURFACES. — 
Different  surfaces  reflect  the  light  that  strikes  them  in  dif 
ferent  degrees.     By  none  is  the  whole  reflected. 

If  any  surface  were  a  perfect  reflector,— that  is,  threw  back  all  the  light 
that  struck  it, — the  eye  would  fail  to  distinguish  it.  Looking  at  such  a  sur- 
face,  we  should  see  nothing  but  images  of  the  bodies  that  produced  the 
incident  rays.  If,  for  example,  the  moon  reflected  all  the  light  it  received, 
it  would  have  the  appearance  of  another  sun.  It  is  because  there  is  not  a 

it  produced  ?  Illustrate  the  mode  in  which  the  shadow  and  penumbra  are  produced, 
with  Fig.  230.  601.  When  light  strikes  an  opaque  body,  what  becomes  of  it  ?  Ex- 
press this  according  to  the  Undulatory  Theory.  To  what  is  the  reflection  of  light 
analogous  ?  How  are  non-luminous  bodies  seen  ?  Is  the  reflection  of  light  con- 
fined to  opaque  surfaces?  Prove  that  it  is  not.  602.  What  is  Catoptrics? 
603.  What  is  meant  by  Incident  Rays?  604.  What  is  said  of  the  reflection  of  light 
from  different  surfaces  ?  If  any  surface  were  a  perfect  reflector,  what  would  be  the 


REFLECTION   OF  LIGHT.  237 

perfect  and  regular  reflection  that  the  non-luminous  bodies  which/  meet  the 
eye  every  moment  are  visible. 

Though  incident  light  is  never  wholly  reflected,  yet  from  some  (surfaces  it 
is  thrown  off  with  a  high  degree  of  regularity,  and  with  its  intensity  dimin- 
ished comparatively  little.  If,  for  instance,  we  look  at  a  good  plate-glass 
mirror  hung  opposite  to  us  at  the  end  of  a  room,  we  can  hardly  persuade 
ourselves  that  there  is  not  another  apartment  beyond,  the  counterparVof  the 
one  which  we  are  in.  The  surface  of  the  mirror  is  not  seen  at  all,  in  cbc  se- 
quence of  its  great  reflective  power. 

605.  The  proportion  of  incident  light  reflected  depends 
on  two  things: — 1.  The  angle  at  which  it  strikes  the  sur- 
face.    2.  The  character  of  the  surface. 

The  more  obliquely  light  strikes  a  surface,  the  greater 
is  the  quantity  reflected. 

In  Fig.  231,  let  C  D  be  a  surface  of  polished  Fig.  231. 

black  marble.  A  and  B  are  incident  beams, 
with  an  intensity  rated  at  1,000.  Let  B  strike 
the  marble  at  an  angle  of  3  degrees,  and  a 
beam  having  an  intensity  of  600  will  be  re- 
flected. Let  A  strike  it  at  an  angle  of  90  de- 
grees, and  the  reflected  beam  will  have  an  intensity  of  only  about  20. 

Light-colored  'and  polished  surfaces  reflect  a  much 
greater  proportion  of  incident  light  than  dark  and  dull 
ones.  Here  again  the  laws  of  light  and  heat  agree. 

A  room  with  white  walls  is  much  lighter  than  one  with  black  or  dark- 
colored  walls.  A  house  painted  some  light  color,  or  a  dome  covered  with 
polished  tin,  is  more  readily  seen  from  a  distance  than  a  dark  wall  or  an  or- 
dinary roof. 

606.  MIRRORS. — The  laws  of  reflected  light  are  best  in- 
vestigated and  explained  with  the  aid  of  mirrors. 

607.  Mirrors  are  solids  with  regular  and  polished  sur- 
faces, having  a  high  degree  of  reflective  power.     They  are 
made  either  of  some  metal  susceptible  of  a  high  polish,  such 
as  silver  and  steel,  or  of  clear  glass  covered  on  the  back 
with  silver  or  a  mixture  of  tin  and  mercury.     A  metallic 
mirror  is  sometimes  called  a  Speculum  (plural,  specula). 

consequence  ?  What  is  said  of  the  reflective  power  of  some  surfaces,  such  as  a  good 
plate-glass  mirror  ?  605.  On  what  does  the  proportion  of  incident  light  reflected  de- 
pond  ?  At  what  angle  is  the  most  incident  light  reflected  ?  Illustrate  this  with  Fig. 
231.  What  sort  of  surfaces  reflect  the  most  incident  light?  606.  With  what  are  the 
la*  a  of  reflected  light  best  investigated  ?  607,  What  are  Mirrors  ?  Of  what  are  they 


238  OPTICS. 

From  glass  mirrors  there  are  two  reflections ;  one  from  the  surface  first 
struck,  the  other  from  the  back  coated  with  mercury.  Hence  two  images  of 
an  object  before  the  mirror  are  presented,  the  distance  between  them  being 
equal  to  the  thickness  of  the  glass.  But  the  image  produced  by  the  front 
surface  is  always  faint ;  and,  when  the  back  is  well  coated,  the  other  image 
is  so  much  superior  that  the  faint  one  is  entirely  lost. 

608.  Kinds  of  Mirrors. — As  regards  shape,  mirrors  are 
divided  into  three  classes  ;  Plane,  Concave,  and  Convex. 

A  Plane  Mirror  (AB,  in  Fig.  232)  is  one  that  reflects 
from  a  flat  surface,  like  a  common  looking-glass. 

A  Concave  Mirror  (E  F,  in  Fig.  233)  is  one  that  reflects 
from  a  curved  surface  hollowing  in  like  the  inside  of  the 
peel  of  an  orange. 

A  Convex  Mirror  (CD,  in  Fig.  234)  is  one  that  reflects 
from  a  curved  surface  rounding  out  like  the  outside  of  an 
orange. 

A  concave  mirror  polished  on  both  sides  becomes  a  convex  mirror  when 
its  opposite  side  is  presented  to  the  incident  rays. 

609.  GREAT  LAW  OF  REFLECTED  LIGHT. — The  law  of 
reflected  light  is  like  that  of  reflected  motion  : — TJie  angle 
of  reflection  is  always  equal  to  the  angle  of  incidence.  This 
law  holds  good  whether  the  reflecting  surface  is  plane,  con- 
cave, or  convex. 

Fig.  232.  Fig.  233.  Fig.  234 


Figs.  232,  233,  234,  illustrate  this  law.  In  each  Figure,  I  represents  the 
incident  ray,  R  the  reflected  ray,  and  P  a  perpendicular.  I Q  P,  the  angle 
which  the  incident  ray  makes  with  the  perpendicular,  is  called  the  angle  of 
incidence.  R  Q  P,  the  angle  which  the  reflected  ray  makes  with  the  same 
perpendicular,  is  the  angle  of  reflection.  From  every  surface,  whatever  its 
form,  the  incident  ray  is  thrown  off  in  such  a  way  as  to  make  the  angle  of 
reflection  equal  to  the  angle  of  incidence. 

made  ?  What  is  a  Speculum  ?  How  many  reflections  are  there  from  glass  mirrors  ? 
How  are  they  produced  ?  What  is  said  of  the  images  formed  ?  608.  As  regards 
shape,  how  are  mirrors  divided  ?  What  is  a  Plane  Mirror  ?  What  is  a  Concave  Mir- 
ror? What  is  a  Convex  Mirror  ?  How  may  a  concave  mirror  polished  on  both  sides 
be  made  a  convex  mirror?  609.  State  the  law  of  reflected  light.  Illustrate  this  with 


FOEMATION   OF  IMAGES. 


239 


610.  From  these  Figures  it  is  obvious  that  an  object  which  would  not 
otherwise  be  visible  can  be  seen  by  reflection  from  a  mirror.  Thus,  let  the 
upper  part  of  P  Q  represent  an  opaque  screen,  I  an  object  on  one  side  of  it, 
and  R  the  eye  of  an  observer  on  the  other.  I  is  not  visible  to  a  person  at  R 
looking  directly  at  it,  on  account  of  the  interposition  of  the  screen ;  but,  as 
the  angle  of  reflection  is  always  equal  to  the  angle  of  incidence,  it  can  be 
seen  from  R  by  looking  at  the  mirror. 

611.  IMAGES. — By  the  Image  of  an  object  is  meant  a 
luminous  picture  of  it  formed  by  rays  proceeding  from  its 
different  points.  An  image  is  said  to  be  inverted  when  it 
represents  its  object  as  upside  down, — that  is,  with  its  low- 
est part  uppermost. 

Fig.  235. 


Fig.  235  illustrates  the  formation  of  an  image.  R  B  represents  a  soldier 
with  a  red  .coat  and  blue  trowsers  standing  in  strong  sunlight  opposite 
the  white  wall  W.  Let  the  shutters  S  S  be  thrown  open,  and  not  only  the 
light  reflected  from  the  person  of  the  soldier,  but  also  other  rays,  enter  the 
apartment,  making  its  light  a  mixture  of  all  colors,  or  white,  in  which  the 
red  and  the  blue  tinge  of  the  dress  are  lost,  and  no  image  is  formed.  Now  let 
the  shutters  S  S  be  closed,  leaving  at  A  an  exceedingly  small  aperture,  through 
which  the  rays  reflected  from  the  figure  are  allowed  to  reach  the  wall.  As 
light  is  propagated  in  straight  lines,  the  ray  R  will  strike  the  wall  at  r,  B  at 
b,  and  I  at  i.  The  image  will  therefore  be  inverted ;  and,  as  each  ray  retains 
its  color,  the  coat  will  remain  red  and  the  trowsers  blue.  This  experiment 
confirms  two  principles  already  stated : — 1.  That  every  ray  moves  in  a 
straight  line ;  2.  That  an  infinite  number  of  rays  may  cross  each  other  with- 
out interfering  with  the  effect  which  each  would  separately  have. 

612.  Images  formed  by  apertures  are  always  inverted. 


the  Figures.  610.  What  is  obvious  from  these  Figures?  611.  What  is  meant  by  the 
Image  of  an  object?  When  is  an  image  said  to  be  inverted?  With  Fig.  235,  illus- 
trate the  formation  of  an  image.  What  two  principles  does  this  experiment  confirm  1 


240  OPTICS. 

613.  REFLECTION  FROM  PLANE  MIRRORS. — Plane  mir- 
rors do  not  alter  the  relative  direction  of  incident  rays.     If 
the  incident  rays  are  parallel,  they  will  remain  parallel  after 
reflection ;  if  divergent,  they  will  continue  to  diverge  ;  if 
convergent,  they  will  continue  to  converge. 

614.  Objects  seen  in  a  plane  mirror  seem  to  lie  in  the 
direction  of  the  reflected  rays  that  meet  the  eye,  and  to  be 
as  far  behind  the  mirror  as  they  really  are  in  front  of  it. 
These  principles  are  illustrated  with  Fig.  236. 

p.  23S  A  B  is  a  plane  mirror.  C,  D,  are  parallel  rays  striking 

its  surface.  They  are*  reflected  in  parallel  lines  to  c,  d't 
and  to  an  observer  at  those  points  will  appear  to  come 
from  G,  H,  as  far  behind  the  mirror  as  C,  D,  are  in  front 
of  it. 

E  is  a  diverging  pencil.  After  reflection,  its  rays  con- 
tinue to  diverge  to  e,  e,  e;  and  to  an  observer  there  they 
appear  to  diverge  in  unbroken  straight  lines  from  the  point 
I,  as  far  behind  the  mirror  as  E  is  before  it. 

F,  F,  F,  represent  converging  rays.-  After  reflection, 
they  continue  to  converge,  and  meet  at  the  point  /.  An 
observer  aty  would  suppose  them  to  come  in  unbroken 
lines  from  J,  J,  J,  as  far  behind  the  mirror  as  F,  F,  F,  are 
in  front  of  it. 

615.  When  we  walk  towards  a  looking-glass,  our  image 
seems  to  advance  towards  us  ;  and  when  we  recede  from 
it,  the  image  also  recedes.  The  image  always  appears  to  be  the  same  dis- 
tance from  the  mirror  that  the  object  is. 

616.  The  angle  of  reflection  being  equal  to  the  angle  of 
incidence,  it  follows  that  a  person  may  see  his  whole  figure 
rig.  237.  reflected   from    a    mirror  whose 

length  is  but  half  his  own  height. 
In  Fig.  237,  CD  represents  a 
man  standing  before  the  mirror 
A  B.     The  incident  ray  from  the 
head  C  strikes  the  mirror  perpen- 
dicularly, is  reflected  in  the  same  line,  and  appears  to  come 


612.  What  kind  of  images  are  formed  by  apertures  ?  613.  What  effect  have  plane 
mirrors  on  the  relative  direction  of  incident  rays  ?  614.  How  do  objects  seen  in 
plane  mirror  seem  to  lie  ?  With  Fig.  236,  illustrate  the  reflection  of  parallel,  diverg- 
ing, and  converging  rays  from  a  plane  mirror.  615.  When  we  approach  and  recede 
from  a  looking-glass,  what  phenomena  are  presented  ?  616.  How  is  it  that  a  person 
can  see  his  whole  figure  reflected  from  a  mirror  whose  length  is  but  half  his  height  ? 


REFLECTION  FROM  PLANE  MIRRORS.  241 

from  E.  The  ray  from  his  foot  D  strikes  the  mirror  at  B, 
is  reflected  at  an  equal  angle  to  his  eye,  and  appears  to 
come  in  an  unbroken  line  from  F.  The  extremities  of  his 
person  being  seen,  the  intermediate  parts  are  also  visible, 
forming  a  complete  image. 

617.  Images  formed  ly  Plane  Mirrors. — The  size  of 
images  formed  by  plane  mirrors  is  not  changed,  except  so 
far  as  they  seem  smaller  in  consequence  of  their  apparent 
distance  behind  the  mirror. 

618.  As  the  image  faces  the  opposite  way  from  the  object,  if  the  mirror  is 
vertical  (that  is,  perpendicular  to  the  floor),  the  right  side  of  the  object  will 
be  the  left  of  the  image,  and  the  left  side  of  the  object  the  right  of  the  image. 
If  a  person  stands  before  a  mirror  with  a  book  in  his  right  hand,  the  book 
seems  to  be  in  the  left  hand  of  his  image ;  and,  if  he  brings  the  printed  page 
near  the  mirror,  he  can  not  read  it,  for  the  reflection  turns  about  both  letters 
and  words,  side  for  side. 

Place  the  same  plane  mirror  in  a  horizontal  position  (that  is,  lay  it  on  the 
floor  with  its  face  up),  and  the  image,  which  before  simply  had  its  sides 
transposed,  now  becomes  inverted,  or  seems  to  stand  on  its  head.  On  the 
same  principle,  a  tree  or  other  object  reflected  from  the  surface  of  a  pond,  is 
inverted. 

619.  The  Kaleidoscope. — When  an  object  is  placed  be- 
tween two  parallel  plane  mirrors,  each  produces  an  image 
of  its  own,  and  reproduces  the  image  reflected  to  it  from 
the  other.  This  image  of  an  image  is  again  reflected  by 
each  to  the  other,  and  thus  a  series  of  images  is  produced, 
till  the  rays  become  so  faint  by  successive  reflections  as  to 
be  no  longer  discernible. 

When  the  mirrors  are  placed  at  right  angles  to  each 
other,  an  object  between  them  forms  three  images, — one 
(produced  by  each  separately,  and  one  by  a  twofold  reflec- 
'tion  from  both.  Placed  so  as  to  form  with  each  other  an 
!  angle  of  60  degrees,  the  two  mirrors  will  produce  five  im- 
;ages;  at  45  degrees,  seven. 

This  principle  is  applied  in  the  Kaleidoscope  {ka-U'-do- 
i*cope],  a  beautiful  toy  invented  by  Sir  David  Brewster. 

617.  What  is  said  of  the  size  of  images  formed  by  plane  mirrors  ?  618.  If  the  mirror 
j  is  vertical,  how  does  the  image  differ  from  the  object  ?  How,  if  the  mirror  is  horizon- 
'tal?  619.  What  takes  place  when  an  object  is  placed  between  two  parallel  plane 
j  mirrors  ?  How  many  images  are  formed  when  the  mirrors  are  placed  at  right  angle* 
11 


242  OPTICS. 

620.  The  kaleidoscope  consists  of  two  narrow  strips  of  glass  running 
lengthwise  through  a  tube,  and  forming  with  each  other  an  angle  of  60  or  45 
degrees.  One  end  of  the  tube,  to  which  the  eye  is  to  be  applied,  is  covered 
with  clear  glass.  The  other  end  terminates  in  a  cell  formed  by  two  parallel 
pieces  of  glass  an  eighth  of  an  inch  apart,  the  outer  one  of  which  is  ground 
to  prevent  external  objects  from  marring  the  effect.  This  cell  contains  beads 
or  small  pieces  of  glass  of  different  colors,  free  to  move  among  themselves. 
On  applying  an  eye  to  the  tube,  we  see  the  objects  in  the  cell  multiplied  by" 
repeated  reflections  from  the  mirrors,  and  symmetrically  arranged, with  their 
images,  around  a  common  centre.  By  shaking  the  tube,  we  bring  the  ob- 
jects into  new  relative  positions,  and  have  new  combinations  presented. 

621.  The  Magic  Perspective. — By  arranging  four  plane 
mirrors  as  represented  in  Fig.  238,  a  person  is  enabled  to 
see  an  object  by  looking  directly  towards  it,  though  an 
opaque  screen  is  interposed. 

A  rectangular  box  is  bent 

Fig-  233.  four  times  at  right  angles; 

and  in  each  of  these  angles 
is  placed  a  piece  of  looking- 
glass,  BK  C,  D,  E,  at  such 
an  inclination  that  the  inci- 
dent ray  may  strike  it  at  an 
angle  of  45  degrees.  Anj; 
THE  MAGIO  PERSPECTITE.  obJfct  opposite  the  aperture 

A  is  visible  to  an  eye    ap- 

-  plied  at  the  other  extremity,  though  an  opaque  screen  be  placed  between  th* 
arms  of  the  instrument.  The  rays  from  the  object  first  strike  B  at  an  angle 
of  45  degrees,  and  are  reflected  at  the  same  angle  to  C,  thence  to  D,  thence  to 
E,  and  finally  to  the  observer's  eye.  The  inventor  of  this  instrument  recom- 
mended its  use  in  time  of  war,  for  discovering  an  enemy's  movements  with* 
out  any  exposure  of  the  observer's  person.  It  is  more  commonly  used,  how* 
evlfr,  by  itinerant  showmen,  who  for  a  penny  allow  the  curious  to  read  through 
a  brick. 

622.  REFLECTION  FROM  CONCAVE  MIRRORS. — In  gen- 
eral, the  effect  of  concave  mirrors  is  to  make  incident  rays 
more  convergent  or  less  divergent.     In  most  cases,  the  im- 
ages they  produce  appear  in  front  of  them. 

623.  Parallel  rays  striking  a  concave  mirror  are  made 
to  converge  to  a  point  called  the  Principal  Focus.     This 

to  each  other  ?  How  many,  when  they  form  an  angle  of  60  degrees  ?  Of  45  degrees? 
In  what  is  this  principle  applied?  620.  Describe  the  Kaleidoscope.  621.  How  is  a 
person  enabled  to  see  an  object  by  looking  towards  it,  though  an  opaque  screen  is  in- 
terposed ?  Describe  the  Magic  Perspective.  By  whom  is  it  commonly  used  ? 
I  622.  What  is  the  general  effect  of  concave  mirrors?  What  is  said  of  the  images  they 


REFLECTION  FROM   CONCAVE  MIRRORS.  243 

point  is  half  way  between  the  surface  of  the  mirror  and  the 
centre  of  the  sphere  which  the  mirror  would  form  if  it  were 
extended  with  uniform  curvature. 

In  Fig.  239,  let  A  E  B  be  a  concave  mir-  Fig.  239. 

ror,  forming  part  of  the  surface  of  a  sphere, 
of  which  C  is  the  centre.  The  parallel  rays 
d,  e>f>  ff>  h>  are  reflected  to  the  principal  fo- 
cus F,  midway  between  the  surface  and  the 
centre  C. 

Not  only  is  light  concentrated  at  the  fo- 
cus,  but  also  heat,  as  we  had  occasion  to 

note  in  §  476.  Tinder,  wood,  or  any  other  combustible  material,  is  readily 
ignited,  and  with  a  combination  of  such  mirrors  the  most  intense  heat  can  be 
produced.  Hence  concave  mirrors  are  sometimes  called  Burning  Glasses. 

C24.  Converging  rays  reflected  from  a  concave  mirror 
are  made  to  converge  more. 

625.  Diverging  rays  reflected  from  concave  mirrors  are 
differently  affected  according  to  the  position  of  the  point 
from  which  they  diverge. 

626.  Diverging  rays  starting  from  the  principal  focus 
are  made  parallel.     This  is  obvious  from  Fig.  239.     The 
rays  diverging  from  F,  after  striking  the  mirror,  are  re- 
flected in  parallel  lines  to  <#,  e,  f,  #,  h. 

This  principle  is  turned  to  account  in  light-houses.  The  light  is  placed  in 
the  focus  of  a  concave  mirror,  and  its  rays  are  reflected  in  parallel  lines  from 
every  point  of  the  mirror's  surface.  No  image  of  the  light  is  produced,  but 
the  whole  surface  of  the  mirror  appears  illuminated. 

627.  Diverging  rays  coming  from  a  point  between  the 
principal  focus  and  the  mirror,  become  less  divergent  after 
reflection.     An  object  in  such  a  position  forms  an  image 
larger  than  itself,  which  seems  to  be  situated  behind  the 
mirror. 

628.  Diverging  rays  coming  from  a  point  between  the 

produce  ?  623.  What  effect  has  a  concave  mirror  on  parallel  rays  that  strike  It  ? 
How  is  the  principal  focus  situated  ?  Illustrate  this  effect  with  Fig.  239.  What  are 
concave-  mirrors  sometimes  called,  and  why  ?  624.  "What  is  the  effect  of  concave  mir- 
rors on  converging  rays  ?  626.  What  is  the  effect  of  concave  mirrors  on  diverging 
rays  starting  from  the  principal  focus?  How  is  this  principle  turned  to  account? 

627.  What  effect  have  concave  mirrors  on  diverging  rays  coming  from  a  point  be- 
tween the  principal  focus  and  the  mirror?    What  kind  of  an  image  is  formed? 

628.  What  effect  have  concave  mirrors  oa  rays  diverging  from  a  point  between  the 


244  OPTICS. 

principal  focus  and  the  centre,  converge,  after  reflection, 
to  a  focus  on  the  other  side  of  the  centre.  An  inverted 
image  will  there  be  visible,  suspended  in  the  air.  This  im- 
age is  made  more  distinct,  and  its  effect  greatly  increased, 
by  causing  a  cloud  of  thin  bluish  smoke  to  rise  about  the 
spot  from  a  chafing-dish  placed  beneath. 

By  concealing  with  screens  the  mirror,  the  object,  and  the  light  that  illu- 
mines it,  and  allowing  the  reflected  rays  to  pass  through  an  aperture,  we  may 
give  the  image  all  the  appearance  of  reality.  The  observer  beholds  delicious 
fruit  hanging  in  the  air  without  any  visible  support,  and  can  hardly  convince 
himself  that  it  is  a  delusion,  even  when  he  tries  to  grasp  it  without  success. 
He  sees  a  pail  full  of  water  standing  bottom  upward  without  spilling  its 
contents,  and  men  with  every  semblance  of  life  walking  on  their  heads.  It 
was  with  apparatus  of  this  kind  that  the  pretended  magicians  of  the  Middle 
Ages  wrought  many  of  their  miracles,  terrifying  the  uninitiated  with  sudden 
apparitions  of  skulls,  drawn  swords,  skeletons,  ghosts,  &c. 

629.  Diverging  rays  coming  from  the  centre  are  reflect- 
ed by  a  concave  mirror  back  to  the  same  point.     Here,  as 
in  all  other  cases,  the  angle  of  reflection  is  equal  to  the  an- 
gle of  incidence.     Striking  the  surface  at  right  angles,  they 
are  reflected  at  right  angles  back  to  the  centre. 

630.  Diverging  rays  coming  from  a  point  beyond  the 
centre,  after  reflection  by  a  concave  mirror,  converge  to  a 
point  on  the  other  side  of  the  centre.     In  this  case,  the  im- 
age is  inverted  and  smaller  than  the  object. 

631.  REFLECTION  BY  CONVEX  MIRRORS. — In  general,  the 
eflect  of  convex  mirrors  is  to  make  incident  rays  more  di- 
vergent or  less  convergent.    The  images  they  produce,  like 
those  of  plane  mirrors,  seem  to  stand  behind  them,  and  are 
generally  smaller  than  the  objects  they  represent. 

632.  Parallel  rays  striking  a  convex  mirror  are  made  to 
diverge,  as  if  they  proceeded  from  a  point  on  the  opposite 
side  of  the  mirror,  called  the  Virtual  Focus.    This  point  is 


principal  focus  and  the  centre  ?  "What  sort  of  an  image  is  formed  ?  How  is  the  image 
made  more  distinct ?  How  may  wonderful  effects  be  produced  with  this  mirror?  By 
whom  was  apparatus  of  this  kind  employed?  629.  What  is  the  effect  of  concave  mir- 
rors on  diverging  rays  coming  from  the  centre  ?  630.  What  is  their  effect  on  diverg- 
ing rays  coming  from  a  point  beyond  the  centre  ?  In  this  case,  what  kind  of  an  image 
is  produced  ?  631.  What  is  the  general  effect  of  convex  mirrors  ?  What  is  said  of 
tne  images  they  produce  ?  632.  What  is  the  effect  of  a  convex  mirror  on  parallel 


REFLECTION   BY   CONVEX   MIRRORS.  245 

half  way  between  the  mirror  and  the  centre  of  the  sphere 
which  the  mirror  would  form,  if  it  were  extended  with  uni- 
form curvature. 

In  Fig.  240,  let  A  B  represent  a 
convex  mirror  forming  part  of  the 
surface  of  a  sphere,  of  which  C  is  the 
centre.  The  parallel  rays  a,  b,  c,  d,  e, 
diverge  after  reflection  to/,  g,  c,  h,  i, 
as  if  they  had  come  from  the  virtual 
focus  F  on  the  other  side  of  the  mir- 
ror. F  is  half  way  between  the  mir- 
ror and  its  centre  C. 

633.  Diverging  rays  fall- 
ing on  a  convex  mirror  are  made  more  divergent  by  reflec- 
tion.    Converging  rays  are  made  less  convergent,  in  some 
cases  even  becoming  parallel. 

Refraction  of  Light. 

634.  When  light  strikes  a  transparent  body,  some  of  it 
is  reflected  and  makes  the  body  visible.     The  rest  enters 
the  body,  and  is  partly  absorbed  and  partly  transmitted 
through  it.   According  to  the  undulatory  theory,  we  should 
say  that  some  of  the  undulations  that  strike  the  transparent 
body  are  reproduced  in  the  same  medium  with  a  change  of 
direction,  while  others  are  brought  to  rest  within  the  bodyt 
and  others  again  are  transmitted  through  it  with  certain 
modifications. 

We  have  treated  of  that  portion  of  the  light  which  is 
reflected  ;  we  must  now  look  at  that  which  enters  the  trans- 
parent body. 

635.  When  a  boy  rowing  a  boat  brings  his  oar  into  the  water,  it  no  longer 
looks  straight,  but  broken  at  the  point  where  it  enters.  The  same  appear- 
ance is  presented  when  he  plunges  a  spoon  or  cane  obliquely  in  a  pail  of  wa- 
ter. On  taking  out  the  oar,  the  spoon,  and  the  cane,  they  look  perfectly 
straight  again.  It  is  evident,  therefore,  that  the  rays  coming  from  the  parts 

rays  ?  "Where  does  the  virtual  focus  lie  ?  Illustrate  the  effect  of  convex  mirrors  on 
parallel  rays,  with  Fig.  240.  633.  What  is  the  effect  of  convex  mirrors  on  diverging 
rays?  On  converging  rays  ?  634.  When  light  strikes  a  transparent  body,  what  be- 
comes of  it  ?  Express  this  according  to  the  Undulatory  Theory.  635.  Give  some  fa- 
miliar examples  which  prove  that  rays  are  bent  on  passing  from  one  medium  to  an- 


246  OPTICS. 

immersed  are  turned  from  their  course  on  entering  the  air,  so  that  the  points 
from  which  they  come  appear  to  lie  where  they  do  not  really  lie.  Rays  thus 
turned  from  their  course  are  said  to  be  refracted. 

636.  Refraction  is  that  change  of  direction  which  a  ray 
of  light  experiences  on  passing  obliquely  from  one  medium 
to  another. 

For  an  example,  see  the  ray  A  in  Fig.  241.  If  there  were  no  water  in  the 
ressel,  it  would  go  on  in  a  straight  line  to  B ;  when  the  vessel  is  filled,  it  is 
refracted  to  C. 

637.  That  branch  of  Optics  which  treats  of  the  laws  and 
principles  of  refracted  light,  is  called  Dioptrics. 

638.  REFRACTIVE  POWER  OP  DIFFERENT  MEDIA. — All 
media  do  not  have  the  same  refractive  power.     Rays  of 
light  falling  from  the  air  on  water,  alcohol,  glass,  and  ice, 
are  turned  from  their  course  in  different  degrees  by  each. 

A  medium  that  has  great  refractive  power  is  said  to  be 
dense;  one  that  has  but  little,  is  called  rare..  The  terms 
dense  and  rare,  therefore,  applied  to  media  hi  Optics,  have 
a  different  meaning  from  that  which  they  convey  in  other 
departments  of  Natural  Philosophy. 

As  a  general  rule,  those  media  are  the  densest  that  have  the  greatest  spe- 
cific gravity ;  and,  of  media  having  about  the  same  specific  gravity,  the  most 
inflammable  is  the  densest.  The  following  substances  are  arranged  accord- 
ing to  their  refractive  power,  chromate  of  lead,  a  transparent  solid,  being  the 
densest : — Chromate  of  lead,  diamond,  phosphorus,  sulphur,  mother-of-pearl, 
quartz,  amber,  plate-glass,  olive  oil,  alcohol,  water,  ice,  air,  oxygen,  hy- 
drogen. 

639.  LAWS  OF  REFRACTED  LIGHT.—!.    In  a  uniform 
medium,  there  is  no  refraction.   It  is  only  on  passing  from 
one  medium  (or  stratum  of  a  medium)  to  another,  that  a 
ray  is  turned  from  its  course. 

2.  Only  such  rays  as  enter  a  medium  obliquely  are  re- 
fracted,— not  such  as  enter  at  right  angles.  . 

3.  ~W7ien  a  ray  passes  obliquely  from  a  rarer  to  a  denser 

other.  What  term  is  applied  to  such  rays?  636.  What  is  Refraction  ?  Illustrate  this 
definition  with  Fig.  241.  637.  What  is  Dioptrics?  638.  What  is  said  of  the  refractive 
power  .of  different  media  ?  What  is  a  Dense  Medium  ?  What  is  a  Rare  Medium  ? 
What  is  said  of  the  meaning  of  the  terms  dense  and  rare  in  Optics  ?  As  a  general 
rule,  what  media  are  the  densest  ?  Mention  some  substances  in  the  order  of  their 
refractive  power  ?  639.  What  is  the  first  law  of  refracted  light  ?  The  second  ?  The 


REFRACTION. 


247 


Fig.  242. 


medium,  it  is  refracted  towards  a  line  perpendicular  to 
the  surface.     In  Fig.  241,  let  the  ray  A  pass  from  air,  a 
rarer  medium,  into  water,  a  denser  medium,  and  instead  of 
going  on  in  a  straight  line  to  B,  it  will  be          Fig.  241. 
refracted  to  C,  nearer  the  perpendicular. 

4.  When  a  ray  passes  from  a  denser  me- 
dium into  a  rarer,  it  is  refracted  from  the 
perpendicular.  In  Fig.  241,  let  the  ray  B 
pass  obliquely  from  water  into  air,  and  in- 
stead of  going  on  in  a  straight  line  to  A,  it 
will  be  refracted  to  D,  farther  from  the  perpendicular. 

640.  An  interesting  experiment  which  every  pupil  may  perform  for  him- 
self, admirably  illustrates  refraction,  and  proves  the  last  law  to  be  true. 
Place  a  coin  on  the  bottom  of  an  empty  vessel  (see 
Fig.  242),  and  fix  the  eye  in  such  a  position  that 
it  just  misses  seeing  it  on  account  of  the  vessel's 
side  coming  between.    Keep  the  eye  there,  and 
let  water  be  poured  in ;  the  coin  will  then  become 
visible,  the  rays  from  its  surface  being  refracted 
so  as  to  meet  the  eye.    The  coin  will  appear  to  lie 
at  N,  some  distance  above  the  bottom  of  the  ves- 
sel ;  because  the  rays  from  it  that  last  meet  the  eye,  if  continued  in  straight 
lines,  would  go  on  to  that  point. 

The  change  caused  by  refraction  in  the  apparent  position  of  an  object 
often  misleads  persons  standing  on  the  bank  of  a  sheet  of  water  as  to  its 
depth.  Objects  on  the  bottom  seem  to  be  several  feet  nearer  the  surface  than 
they  are,  and  bathers,  deceived  by  the  appearance,  venture  beyond  their 
depth  and  are  drowned. 

641.  ATMOSPHERIC  REFRACTION. — Rays  from  the  heav- 
enly bodies,  on  entering  our  atmosphere  obliquely  from  a 
rarer  medium,  are  refracted  towards  the  perpendicular. 
Hence  we  never  see  these  bodies  in  their  real  position,  ex- 
cept when  they  are  directly  over  head. 

The  sun  is  visible  to  us  some  time  before  he  really  rises  above  the  horizon, 
and  remains  visible  at  night  after  he  has  sunk  below  it.  We  owe  our  twi- 
light to  successive  reflections  and  refractions  of  his  rays  by  atmospheric 
strata  of  different  densities,  after  he  has  disappeared. 

third  ?  The  fourth  ?  Illustrate  the  third  and  the  fourth  law  with  Fig.  241.  640.  What 
Interesting  experiment  illustrates  refraction  ?  How  are  persons  standing  on  the  bank 
of  a  sheet  of  water  often  deceived  ?  641.  When  do  we  see  the  heavenly  bodies  in 
their  real  position  ?  Why,  at  other  times,  do  we  not  see  them  in  their  real  position  ? 


248  OPTICS. 

642.  Mirage.  —  Different  strata  of  the  atmosphere  differ 
in  their  refractive  power.  Accordingly,  rays  from  an  olx 
ject  below  the  horizon  (that  is,  concealed  from  us  by  the 
roundness  of  the  earth)  may,  under  peculiar  circumstances, 
by  successive  refractions  through  different  strata,  be  made 
to  describe  a  curve  to  our  eyes,  and  will  in  that  case  ap- 
pear to  come  from  a  distant  point  in  the  air  lying  in  the 
direction  of  the  line  described  by  the  ray  as  it  entered  the 
eye.  Such  is  the  origin  of  the  phenomenon  called  Mirage 


Mirage  is  the  appearance  in  the  air  of  an  erect  or  in- 
verted image  of  some  distant  object  which  is  itself  invisible. 
It  is  most  frequently  seen  on  the  water,  but  has  also  ap. 
peared  to  persons  travelling  through  deserts,  with  such  viv- 
idness as  to  make  them  believe  that  they  saw  trees  and 
springs  before  them  in  the  distance. 

Mirage  is  sometimes  remarkably  distinct  at  sea.  Cap.tain  Scoresby,  on 
one  occasion,  in  a  whaling-ship,  recognized  his  father's  vessel,  when  distant 
from  him  more  than  30  miles  (and  consequently  below  the  horizon),  by  its 
inverted  image  in  the  air,  though  he  did  not  previously  know  that  it  was 
cruising  in  that  part  of  the  ocean.  Another  notable  case  occurred  on  the 
coast  of  Sussex,  England.  Cliffs  were  distinctly  seen  in  the  air  ;  and  the 
sailors,  crowding  to  the  beach,  recognized  different  parts  of  the  French  shore, 
distant  from  40  to  50  miles.  These  phenomena  are  comparatively  frequent 
in  the  Strait  of  Messina,  and  as  there  exhibited  have  been  called  Fata  Mor- 
gana [fah'-tah  mor-ffah'-nah]. 

643.  REFRACTION  BY  PRISMS  AND  LENSES.  —  Prisms  and 
lenses  are  much  used  in  experimenting  on  light  and  in  the 
construction  of  optical  instruments. 

Fi?.  243.  644-  -Prisms.  —  A  Prism  (see  Fig.  243) 

A  —  /\    is  a  solid  piece  of  glass,  having  for  its  sides 

XI  \|    three  plane  surfaces  and  for  its  ends  two 

A  PRISM.  equal  and  parallel  triangles. 

645.  A  ray  of  light  falling  on  a  prism  must  pass  through 

two  of  its  surfaces.     If  it  strike  both  of  them  obliquely,  it 

To  what  do  we  owe  our  twilight  ?  642.  Explain  how  an  object  below  the  horizon  is 
Tendered  visible.  What  phenomenon  is  thus  produced  ?  What  is  Mirage  ?  Where 
is  it  seen  ?  What  case  of  mirage  is  recorded  by  Captain  Scoresby  ?  What  other  nota- 
ble case  is  mentioned  ?  Where  are  these  phenomena  frequent  ?  643.  What  are  much 
used  in  experimenting  on  light  ?  644,  What  is  a  Prism  ?  645.  What  is  the  effect  ol 


REFRACTION  BY   PRISMS   AND    LENSES, 


249 


Pig.  244. 


will  be  twice  refracted ;  if  it  strike  one  surface  perpendic- 
ularly and  the  other  obliquely,  it  will  be  refracted  but  once. 
In  either  case,  the  object  from  which  it  comes  will  appear 
to  lie  in  a  position  more  or  less  removed  from  its  real  one. 

Fig.  244  shows  the  refractive  effect  of  a  prism. 
A  ray  from  E,  entering  the  prism  ABC,  from 
air,  a  rarer  medium,  is  refracted  to  D,  and  on 
passing  back  into  the  rarer  medium,  at  that  point 
is  refracted  to  the  eye.  The  object  from  which  it 
comes  appears  to  lie  at  F,  in  the  direction  from 
which  the  ray  entered  the  eye.  Had  there  been 

but  one  refraction,  it  would  still  have  appeared  elevated  above  its  real  posi- 
tion, but  not  so  much. 

646.  Lenses. — A  lens  is  a  transparent  body  which  has 
two  polished  surfaces,  either  both  curved  or  one  curved  and 
the  other  plane.     The  general  effect  of  lenses  is  to  refract 
rays  of  light,  and  magnify  or  diminish  objects  seen  through 
them.     They  are  generally  made  of  glass ;  but  in  specta- 
cles rock  crystal  is  sometimes  used  instead  of  glass,  because 
it  is  harder  and  less  easily  scratched. 

647.  Classes  of  Lenses. — Lenses  are  divided  into  six 
classes  according  to  their  shape.     Fig.  245  shows  these  six 
classes.     The  name  of  each  is  given  on  one  side,  and  a  de- 
scription of  it  on  the  other. 

Fig.  245. 


DOUBLE  CONVEX  LENS. 
PLANO-CONVEX  LENS. 

MENISCUS. 

DOUBLE  CONCAVE  LENS. 

PLANO-CONCAVE  LENS. 
COXCAVO-CONVEX  LENS. 


Both  sides  convex. 


One  side  convex,  the  other  plane. 

(  One  side  convex,  the  other  concave. 
1  Thickest  in  the  middle. 


Both  sides  concave. 

One  side  concave,  the  other  plane. 

j  One  side  concave,  the  other  convex. 
\  Of  uniform  thickness,  or  thickest  at  tha 
ends. 


a  prism  on  a  ray  of  light  ?  Show  this  effect  with  Fig.  244.  646.  What  is  a  lens  ? 
What  is  the  general  effect  of  lenses  ?  Of  what  are  they  made  ?  647.  Into  how  many 
classes  are  lenses  divided  ?  Name  them.  Describe  the  Double  Convex  Lens.  The 
Plano-convex.  The  Meniscus.  The  Double  Concave  Lens.  The  Plano-concave. 


250  OPTICS. 

The  first  three  of  the  above  lenses,  which  are  thickest  in  the  middle,  are 
called  Convex  Lenses,  and  their  effect  is  to  make  rays  passing  through  them 
incline  more  towards  each  other.  The  next  two  (the  double  concave  and 
plano-concave)  which  are  thinnest  in  the  middle,  are  called  Concave  Lenses, 
and  their  effect  is  to  make  rays  passing  through  them  incline  farther  from 
each  other. 

The  concavo-convex  lens,  when  its  two  surfaces  are  parallel  (as  in  the 
above  Figure)  does  not  change  the  direction  of  rays  passing  through  it,  for 
the  convergent  effect  of  the  convex  surface  is  nullified  by  the  divergent  effect 
of  the  concave  surface.  When  the  convex  surface  has  a  greater  curvature 
than  the  concave,  this  lens  becomes  a  meniscus.  When  the  concave  surface 
has  the  greater  curvature,  it  becomes  a  concave  lens,  and  participates  in  the 
properties  of  that  class. 

648.  Refraction  by  Convex  Lenses. — The  general  effect 
of  convex  lenses  is  threefold: — 1.  They  make  rays  passing 
through  them  incline  more  towards  each  other  than  before. 
2.  They  enable  us  to  see  objects  which  are  invisible  to  the 
naked  eye  on  account  of  their  distance.     3.  They  magnify 
objects  seen  through  them. 

649.  A  double  convex  lens  of  glass,  with  sides  equally 
convex,  brings  parallel  rays  passing  through  it  to  a  focus  at 
the  centre  of  the  sphere,  of  which  the  surface  of  the  lens 
first  struck  by  the  rays  forms  a  part.     This  is  shown  in  Fig. 
246.     Converging  rays  would  be  brought  to  a  focus  be- 
tween  the  centre  and  the  lens  ;  diverging  rays,  on  the  other 
side  of  the  centre. 

Fig.  246.  Fig.  247. 


The  Concavo-convex.  What  are  the  first  three  of  these  lenses  called  ?  What  is  their 
effect  ?  What  are  the  double  concave  and  the  plano-concave  lens  called  ?  What  is 
their  effect  ?  What  is  the  effect  of  the  concavo-convex  lens,  when  its  two  surfaces  are 
parallel?  When  the  convex  surface  has  a  greater  curvature  than  the  concave? 
When  the  concave  surface  has  a  greater  curvature  than  the  convex  ?  648.  What  is 
the  general  effect  of  convex  lenses  ?  649.  What  is  the  effect  of  a  double  convex  glass 
lens  on  parallel  rays  passing  through  it  ?  On  converging  rays ?  On  diverging  rays? 


BEFKACTION   BY   LENSES.  251 

A  plano-convex  lens  brings  parallel  rays  to  a  focus  at  a 
distance  from  the  lens  about  equal  to  the  diameter  of  the 
sphere  of  which  the  convex  surface  of  the  lens  forms  a  part. 
This  is  shown  in  Fig.  247. 

650.  Convex  lenses  collect  heat  as  well  as  light  at  their  focus.  Hence 
they  are  sometimes  called  Burning  Glasses.  Hold  an  old  person's  eye-glass 
in  the  sun-shine  a  short  distance  from  your  hand.  A  bright  spot  of  light 
marks  the  focus,  and  the  heat  at  that  point  soon  becomes  too  great  to  be 
borne.  All  the  rays  that  fall  on  the  surface  of  the  lens  being  concentrated 
in  this  one  point,  the  heat  at  the  focus  is  as  many  times  greater  than  the  heat 
of  ordinary  sun-light  as  the  area  of  the  lens  is  greater  than  the  area  of  the  fo- 
cus. If  the  area  of  the  lens  be  100  square  inches,  and  that  of  the  focus  */4  of 
an  inch,  the  ordinary  heat  of  the  sun  will  be  increased  400  times. 

651.  The  second  effect  of  convex  lenses  follows  from  the 
first.     Light,  it  will  be  remembered,  diminishes  in  intensity 
according  to  the  square  of  the  distance  from  the  luminous 
body ;  hence  rays  from  exceedingly  remote  stars  become 
BO  faint  by  the  time  they  reach  the  eye  as  not  to  produce 
the  sensation  of  vision.   A  convex  glass  concentrates  a  great 
number  of  these  faint  rays,  and  thus  renders  the  distant 
object  visible  to  an  eye  placed  at  its  focus. 

652.  The  third  effect  of  convex  lenses  is  to  magnify  ob- 
jects seen  through  them.    Hence  they  are  sometimes  called 
Magnifying  Glasses.     The  glasses  used  by  old  persons,  as 
well  as  by  engravers  and  others  who  have  to  deal  with  mi- 
nute objects,  are  convex  lenses. 

653.  Refraction  by  Concave  Lenses. — The  effects   of 
concave  lenses  are  opposite  to  those  of  convex.     1.  They 
make  rays  passing  through  them  incline  farther  from  each 
other.     2.  They  diminish  objects  seen  through  them. 

654.  All  the  above  laws  relating  to  prisms  and  lenses  apply  to  rays  pass- 
ing into  them  from  a  rarer  medium,  such  as  air.  If  they  come  from  a  denser 
medium,  the  results  will  be  reversed, — convex  lenses  will  have  a  diverging 
and  diminishing  effect,  while  concave  lenses  will  have  a  converging  and 
magnifying  effect. 

What  is  the  effect  of  a  plano-convex  lens  on  parallel  rays  ?  650.  What  are  convex 
lenses  sometimes  called,  and  why  ?  How  may  their  concentration  of  heat  be  shown  ? 
How  does  the  heat  at  the  focus  compare  with  that  of  ordinary  sun-light  ?  651.  Show 
how  %  convex  lens  enables  us  to  see  distant  heavenly  bodies  that  would  otherwise  be 
Invisible.  652.  What  is  the  third  effect  of  convex  lenses  ?  What  are  they  sometimes 


252 


OPTICS. 


Fig.  248. 


655.  Glasses  with  Parallel  Surfaces. — When  rays  pass  through  a  refracting 
medium  having  parallel  surfaces,  they  leave  it,  not  exactly  in  the  same  line, 
but  in  a  direction  parallel  to  that  in  which  they  entered  it.  The  last  refrac- 
tion nullifies  the  change  of  direction  produced  by  the  first.  Hence  we  see 
objects  through  a  pane  of  window-glass  very  nearly  in  their  real  position.  Ir- 
regularities in  the  glass  cause  objects  seen  through  it  to  look  distorted. 

656.  The  Multiplying  Glass. — If  a  planoconvex  lens 
have  its  convex  surface  ground  into  several  flat  surfaces,  an 
object  seen  through  it  will  be  multiplied  as  many  times  as 
there  are  flat  surfaces. 

In  Fig.  248,  A  B  represents  a  multiplying  glass,  and 
D  an  object  viewed  through  it.  The  ray  D  C,  striking 
both  surfaces  perpendicularly,  reaches  the  eye  without 
refraction  ;  but  D  I  and  D  F,  falling  obliquely,  suffer 
two  refractions,  which  bring  them  also  to.  the  eye  at 
the  focus.  As  objects  are  always  seen  in  the  direction 
in  which  their  rays  enter  the  eye,  three  objects  like  D 
will  be  visible :  one  at  D,  in  its  real  position ;  the 
others,  in  the  direction  of  the  dotted  lines,  at  Gr  and  H. 

MULTIPLYING  65^    DOUBLE    REFRACTION.  —  Certain 

GLASS.  substances  (chiefly  minerals)  have  the  prop- 

erty of  causing  rays  which  pass  through  them  to  take  two 
distinct  paths,  and  thus  produce  two  images.  This  phe- 
nomenon is  called  Double  Refraction. 

Fig.  249.  A  crystal  of  carbonate  of  lime, 

commonly  called  Iceland  Spar,  is 
one  of  the  best  substances  for  ex- 
hibiting double  refraction.  Let  it  be 
placed  over  a  piece  of  paper  con- 
taining lines,  and  each  line  will  be 
seen  double,  as  shown  in  Fig.  249. 

Keeping  the  same  side  on  the 
paper,  and  turning  the  crystal  round 
on  its  axis,  we  find  that  the  double 
lines  continue  parallel,  but  that  the 
distance  between  them  varies, — diminishing  till  they  coincide,  then  increas- 
ing ;  then  diminishing  till  they  coincide  again,  and  then  once  more  increas- 

called  in  consequence  ?  653.  What  are  the  general  effects  of  concave  lenses  ?  654.  In 
what  case  do  the  above  laws  relating  to  prisms  and  lenses  apply  ?  Suppose  the  rays 
pass  into  them  from  a  denser  medium,  what  will  be  the  result  ?  655.  What  effect  has 
a  refracting  medium  with  parallel  surfaces  on  incident  rays  ?  How  do  we  see  objects 
through  a  pane  of  window-glass  ?  656.  How  is  the  multiplying  glass  formed  ?  How 
many  times  is  an  object  seen  through  it  multiplied?  Show  this  with  Fig.  243. 
157.  What  is  Double  Kcfraction  ?  How  is  it  exhibits  1  with  Iceland  spar  ?  What  phe- 


POLARIZATION   OF  LIGHT.  253 

jng.  During  each  revolution  of  the  crystal,  the  lines  will  coincide  twice.  A 
single  pencil  of  rays  is  thus  refracted  into  two  distinct  pencils,  one  of  which, 
following  the  usual  law  of  refraction,  is  called  the  Ordinary  Pencil,  while  the 
other,  deviating  from  that  law,  is  called  the  Extraordinary  Pencil. 

Polarization  of  JLiglit. 

658.  Light  is  said  to  be  polarized,  when,  on  being  re- 
flected or  refracted  by  a  surface  which  it  strikes  at  a  cer- 
tain angle,  it  is  absorbed  by  a  similar  surface  perpendicular 
to  the  former  one,  though  it  is  reflected  or  transmitted  by 
one  forming  any  other  angle  with  it. 

Let  A  and  B  (Fig.  250)  be  two  tubes  open  at  both  pj(T  250 

ends,  and  so  adjusted  to  each  other  that  B  turns  stiff- 
ly within  A.     In  each  tube  fix  a  piece  of  polished  A         B        ^Lt 
glass,  M,  N,  roughened  and  blackened  on  the  back, 
so  as  to  form  an  angle  of  33  degrees  with  the  axis  of       / 
the  tubes.    Bring  the  instrument  into  such  a  position 

that  the  light  from  a  luminous  body,  falling  on  M,  may  be  reflected  along  the 
axis  and  strike  N.  Now,  keeping  the  tube  A  stationary,  turn  within  it  the 
tube  B,  carrying  the  reflector  N.  The  reflection  from  N,  if  observed,  will  be 
seen  to  keep  varying  in  intensity.  In  the  two  positions  in  which  N  is  paral- 
lel to  M,  the  reflection  will  be  brightest ;  at  the  points  midway  between  these, 
— that  is,  when  N  is  perpendicular  to  M, — there  is  no  reflection  at  all.  We 
express  this  by  saying  that  the  light  reflected  from  M  is  polarized. 

659.  The  polarizing  angle, — that  is,  the  angle  which  the 
incident  ray  must  make  with  a  perpendicular  to  the  first 
reflecting  surface,  in  order  to  be  polarized, — is  different  in 
the  case  of  different  substances.     For  glass,  it  is  about  57 
degrees. 

660.  If  a  polarized  ray  be  received  on  a  crystal  of  Ice- 
land spar,  there  will  be  but  a  single  refraction. 

661.  Light  is  polarized  by  reflection  at  a  certain  angle,  as  we  have  just 
seen  ;  by  transmission  through  substances  that  have  the  property  of  double 
refraction, — through  some  imperfectly  crystallized  substances,  such  as  agate, 
mother-of-pearl,  &c., — and  also  through  a  sufficient  number  of  uncrystallized 
plates.  However  produced,  polarized  light  always  has  the  same  properties. 
Its  phenomena  are  striking,  and  seem  to  prove  the  truth  of  the  undulatory 

nomena  are  presented  as  the  crystal  is  turned  around  ?  "What  are  the  two  pencils 
presented  to  the  eye  called?  658.  When  is  light  said  to  be  polarized  ?  Illustrate  the 
polarization  of  light  with  Fig.  250.  659.  What  is  meant  by  the  polarizing  angle? 
What  is  this  angle  in  the  case  of  glass?  660.  If  a  polarized  ray  is  received  on  a  crys- 
tal of  Iceland  spar, -what  follows?  661.  Mention  the  different  ways  in  which  jight  is 
polarized.  What  is  said  of  the  properties  and  phenomena  of  polarized  light,  how- 


254 


OPTICS. 


theory.  It  is  thought  that  the  undulations  of  ether  ordinarily  take  place  in 
planes  perpendicular  to  the  direction  in  which  they  are  propagated;  but 
that,  when  light  is  polarized,  they  take  place  in  planes  parallel  to  this  direc- 
tion. At  certain  angles,  the  undulations,  thus  changed  from  their  usual  di- 
rection, are  reproduced  or  transmitted  by  the  second  reflecting  or  refracting 
surface,  and  reach  the  eye ;  but,  when  the  two  surfaces  form  an  angle  of  90 
degrees,  they  are  stopped,  and  the  sensation  of  vision  is  not  produced. 

662.  The  mineral  called  Tourmaline  [toor' -ma-leen]  pos-( 
sesses  the  property  of  polarizing  light  in  a  high  degree.  It 
is  cut  into  plates  one-twentieth  of  an  inch  thick,  which  are 
fixed  between  plates  of  glass  for  convenience  of  use.  If  we 
look  at  the  sun  through  such  a  plate,  we  shall  find  that  most 
of  the  light  is  transmitted.  Place  a  second  plate  behind 
the  first  and  parallel  to  it,  and  the  light  will  still  be  trans- 
mitted ;  but  turn  the  second  plate  so  as  to  bring  it  at  right 
angles  to  the  first,  and  no  light  will  pass  through. 

663.  Some  crystals  viewed  by  polarized  light,  exhibit  systems  of  beautiful 
rings,  like  those  shown  in  Fig.  251.    Plates  of  the  mineral  called  Selenite, 

Fig.  251. 


bearing  different  designs,  placed  so  as  to  be  seen  by  polarized  light,  display 
the  most  gorgeous  coloring,  and  may  be  made  to  undergo  remarkable  and 
beautiful  changes  by  causing  one  of  the  reflecting  surfaces  to  revolve. 

Chromatics. 

664.  Chromatics  is  that  branch  of  Optics  which  treats 
of  colors. 


ever  it  is  produced?  Explain  the  polarization  of  light  according  to  the  undulatory 
theory.  662.  What  mineral  possesses  the  property  of  polarizing  light  in  a  high  de- 
gree ?  How  is  tourmaline  prepared  ?  What  experiment  may  be  performed  with  tour* 
maline  plates  ?  663.  What  phenomena  are  seen  when  certain  crystals  are  viewed 


THE   SOLAR   SPECTRUM. 


255 


665.  THE  SOLAR  SPECTRUM. — If  a  ray  from  the  sun  be 
admitted  into  a  dark  room  through  a  small  aperture,  it  will 
form  a  circular  spot  of  white  light  on  the  surface  receiving 
it.  But  if,  after  entering  the  room,  it  be  received  on  a 
prism,  as  shown  in  Fig.  252,  it  will  be  decomposed  into 

Fig.  252. 


THE  SOLAR  SPECTRUM. 


seven  different  colors.  When  made  to  fall  on  a  white  sur- 
face, these  seven  colors  are  distinctly  seen,  covering  an 
obloftg  space,  which  is  called  the  Solar  Spectrum  (plural, 
spectra).  They  are  known  as  the  Primary  Colors,  and  in 
every  spectrum  they  are  arranged  in  the  order  shown  in 
the  Figure.  By  combining  the  primary  colors  in  different 
proportions,  other  colors  are  produced. 

The  seven  colors,  it  will  be  observed,  do  not  occupy 
equal  spaces  of  the  spectrum.  Violet  covers  the  greatest 
part,  more  than  one-fifth  of  the  whole ;  and  orange  the 
least,  less  than  one-thirteenth  of  the  whole. 

666.  Ordinary  sun-light  (and  all  white  light)  is  therefore  composed  of 
Seven  colors  combined  in  different  proportions.  In  further  proof  of  this,  we 
may  re-unite  the  seven  primary  colors  of  the  spectrum,  and  we  shall  have 
simply  a  small  circular  spot  of  white  light.  To  re-unite  the  colors,  we  may 
receive  the  spectrum  on  a  concave  mirror  or  double  convex  lens,  which  brings 
together  at  its  focus  the  parts  of  the  decomposed  ray.  Or,  we  may  receive 
the  spectrum  on  another  prism  placed  in  contact  with  the  first,  as  shown 
in  Fig.  252.  In  either  case,  we  have  the  same  circular  spot  of  white  light 
that  would  have  been  formed  if  the  ray  had  not  been  decomposed  at  all. 


by  polarized  light?  When  plates  of  selenite  are  viewed  by  polarized  light? 
664.  What  is  Chromatics  ?  665.  Describe  the  solar  spectrum,  and  the  way  in  which 
It  is  formed.  Name  the  seven  primary  colors  in  order.  How  are  the  other  col- 
ors produced  ?  Which  color  occupies  most  of  the  spectrum,  and  which  the  least  ? 
6G6.  Of  what,  then,  is  all  white  light  composed?  What  further  proof  have  wi 


256  OPTICS. 

We  may  produce  white  light  by  combining  the  seven  primary  colors  in 
another  way.  Divide  the  surface  of  a  circular  card  into  seven  parts  propor- 
tioned to  each  other  as  the  spaces  which  the  different  colors  occupy  in  the 
spectrum,  and  paint  them  the  corresponding  shades.  Then  cause  the  card 
to  revolve  rapidly.  No  separate  color  will  be  visible,  but  the  whole  card 
will  look  white. 

667.  A  prism  decomposes  white  light  into  its  seven  component  parts,  be- 
cause these  parts  are  refracted  differently,  some  more  and  some  less.  It  will 
be  observed  that  red,  which  occupies  the  lowest  part  of  the  spectrum,  is 
turned  from  its  course  the  least ;  orange,  a  little  more ;  yellow,  still  more  ; 
then  green ;  then  blue ;  then  indigo  ;  while  violet,  which  is  at  the  top  of  the 
spectrum,  is  refracted  the  most.  The  colors,  therefore,  have  different  de- 
grees of  refrangibility.  This  fact  was  discovered  by  Sir  Isaac  Newton. 

668.  DIFFERENCE  OF  COLOR,  EXPLAINED.  —  According 
to  the  Undulatory  Theory,  the  color  of  light  depends  on 
the  size  of  the  minute  waves  that  produce  it.     The  undula- 
tions that  excite  in  the  eye  the  sensation  of  red  light  are 
each  fo^-o  of  an  inch  in  breadth  ;  those  that  produce  vio^ 
let,  g-oio  o  >  while  the  intermediate  colors  are  produced  by 
undulations  varying  between  these  limits. 

669.  Color  is  not  a  property  inherent  in  bodies,  but  in 
the  light  that  they  reflect.     A  non-luminous  body  seems  to 
be  whatever  color  it  reflects  to  the  eye. 

An  object  lying  in  green  light,  looks  green ;  in  red  light,  red,  &c.  This 
is  because  green  or  red  is  the  only  light  that  falls  upon  it,  and  therefore  it 
can  reflect  no  other  to  the  eye.  A  body  seen  by  ordinary  light  looks  green, 
when  it  absorbs  all  or  most  of  the  other  colors  of  the  spectrum,  and  reflects 
or  transmits  green  alone.  It  looks  red  when  it  absorbs  the  other  colors,  and 
reflects  or  transmits  red,  &c.  It  looks  white,  when  it  does  not  decompose  the 
light  that  falls  on  it,  but  reflects  all  the  colors  combined.  It  looks  black, 
when  it  absorbs  nearly  all  the  light  that  falls  on  it,  and  does  not  reflect  any 
particular  color  in  preference  to  the  rest. 

670.  What  colors  a  substance  absorbs  and  what  it  reflects,  depends  chieflj 
)n  its  structure.  The  particles  of  some  bodies  are  so  arranged  as  to  hav« 
i  peculiar  affinity  for  certain  colors  ;  these  they  absorb,  reflecting  the  rest. 

of  this  ?  How  may  we  re-unite  the  seven  primary  colors  ?  What  other  mode  U 
.there  of  doing  this  ?  667.  To  what  is  it  owing  that  a  prism  decomposes  white  light 
into  its  seven  component  parts?  By  whom  was  this  fact  discovered?  668.  Ac. 
cording  to  the  Undulatory  Theory,  on  what  does  the  color  of  light  depend?  What 
is  the  difference  in  the  undulations  that  respectively  produce  red  and  violet  light  1 
609.  In  what  is  the  property  of  color  inherent  ?  Why  does  an  object  lying  in  green 
light  look  green?  When  does  an  object  seen  by  ordinary  light  look  green  ?  Whea 
iocs  It  look  white  ?  When,  black  ?  670.  What  is  it  that  determines  what  colors  4 


COMPLEMENTARY   COLOES.  257 

Changes  of  color  are  caused  by  changes  of  structure.  We  may  show  this 
by  au  experiment  with  a  substance  called  iodide  of  mercury.  This  mineral 
is  a  bright  scarlet;  when  heated  and  allowed  to  cool  undisturbed,  it  be- 
comes yellow ;  but,  the  moment  the  surface  is  scratched,  the  particles  re- 
arrange themselves,  and  the  color  turns  back  to  scarlet.  Here  the  same 
particles  undergo  a  marked  change  of  color  by  simply  being  made  to  assume 
a  different  arrangement. 

671.  COMPLEMENTARY   COLORS. — Any  two   colors  are 
said  to  be  Complementary,  when,  if  combined  in  due  pro- 
portion, they  will  produce  white.     Those  colors  are  com- 
plementary to  each  other  which  are  distant  half  the  length 
of  the  spectrum  ;  as, 

Red  and  green,  Orange  and  blue, 

Yellow  and  violet,  White  and  black. 

It  is  a  curious  fact  that  if  we  look  intently  at  a  bright  object  of  any  given 
color  and  then  close  our  eyes,  we  shall  still  see  it,  but  tinged  with  the  com- 
plementary color.  After  gazing  a  few  moments  at  a  bright  fire,  everything 
we  look  at  seems  to  have  a  greenish  hue.  If  we  place  a  red  wafer  on  a  piece 
of  white  paper  and  look  at  it  intently,  we  shall  soon  see  a  circle  of  light  green 
playing  around  it.  A  blue  wafer  will  have  a  similar  circle  of  orange,  and  a 
yellow  wafer  one  of  a  violet  tinge. 

672.  A  color  appears  to  the  best  advantage,  when  placed 
beside  its  complementary  color. 

Thus  red  is  set  off  by  green ;  blue,  by  orange,  Ac.  A  pale  face  appears 
paler  still  when  a  black  dress  is  worn.  On  white  paper,  black  ink  is  plainer 
and  pleasanter  to  the  eye  than  ink  of  any  other  color.  In  arranging  bou- 
quets, and  selecting  different  articles  of  dress  that  are  to  be  worn  together, 
the  effect  of  each  individual  color  is  heightened  by  bringing  it  in  immediate 
contrast  with  its  complementary  color. 

673.  PROPERTIES  OP  THE  SPECTRUM. — Every  ray  of  or- 
dinary sun-light  appears  to  have  three  distinct  properties  : 
— 1.  Brightness.     2.  Heat.     3.  Power  of  producing  chem- 
ical effects.     This  last  property  is  called  Actinism. 

674.  The  chemical  effects  of  sun-light  are  shown  in  various  ways.  Phos- 
phorus and  nitrate  of  silver  undergo  a  marked  change  when  exposed  to  the 

substance  absorbs,  and  what  it  reflects?  By  what  are  changes  of  color  caused? 
Prove  this  with  an  experiment.  671.  When  are  two  colors  said  to  be  Complementa- 
ry? Name  four  pairs  of  complementary  colors.  What  curious  fact  is  stated  with 
respect  to  complementary  colors  ?  Give  examples.  672.  When  does  a  color  appear 
to  the  best  advantage?  Give  examples.  673.  How  many  distinct  properties  has 
every  ray  of  ordinary  sun-  light?  Name  them.  674.  Instance  some  of  the  chemical 


258  OPTICS. 

solar  rays.  Daguerreotypes  and  photographs  are  taken  by  means  of  the  ac- 
tion of  light  on  sensitive  chemical  preparations.  Almost  all  the  colored  vege- 
table juices,  when  exposed  to  sun-light,  undergo  a  change  of  hue.  Hydrogen 
and  chlorine,  which  may  be  mixed  without  danger  in  the  dark,  combine  with 
a  loud  explosion  in  the  light.  Light,  also,  is  essential  to  the  chemical  changes 
which  result  in  the  healthy  growth  of  plants.  Hence  plants  kept  in  a  dark 
room  become  pale  and  sickly.  A  similar  effect  is  produced  on  persons  kept 
away  from  the  light  of  the  sun. 

6V5.  Ordinary  sun-light  combines  these  three  properties, 
but  the  seven  colors  into  which  it  is  decomposed  by  the 
prism  do  not  possess  them  alike.  Brightness  belongs  par- 
ticularly to  yellow ;  heat,  to  red ;  actinism,  to  violet  and 
indigo. 

An  object  that  is  bright  yellow  makes  a  more  vivid  impression  on  the  eye 
than  one  of  any  other  color.  Hence  soldiers  dressed  in  yellow  are  more  dis- 
tinct objects  of  aim  to  an  enemy  and  more  apt  to  be  shot  than  those  dressed 
in  dark  green  or  gray. 

The  red  portion  of  the  spectrum  has  the  most  heat.  This  is  shown  by 
placing  the  bulb  of  a  thermometer  successively  in  each  of  the  colors  of  the 
spectrum.  It  will  be  most  affected  by  the  red,  but  will  show  a  still  higher 
temperature,  if  brought  a  short  distance  below  the  red  end  of  the  spectrum, 
where  no  light  falls  at  all.  This  shows  that  the  heat  of  a  solar  ray  is  re- 
fracted as  well  as  its  light,  but  in  a  less  degree. 

Actinism  is  strongest  in  violet  and  indigo  rays.  If  a  seed  be  placed  un- 
der a  dark  blue  glass,  so  that  all  the  light  that  strikes  it  will  be  tinged  with 
that  color,  it  will  germinate  in  one-fourth  of  the  time  that  it  usually  takes. 
Placed  under  a  red  glass,  it  will  hardly  germinate  at  all,  because  red,  al- 
though it  contains  more  heat  than  the  other  colors,  has  little  or  no  actinism. 

676.  DARK  LINES  IN  THE  SPECTRUM. — If  the  solar  spec- 
trum be  viewed  through  a  telescope,  a  great  number  of 
dark  lines,  parallel  to  each  other  but  differing  in  breadth, 
will  be  seen  crossing  its  surface.  Seven  of  these  are  par- 
ticularly distinct,  but  with  a  powerful  telescope  as  many 
as  2,000  have  been  counted. 

The  position  of  these  lines  is  always  the  same  in  the  solar  spectrum ; 
but,  when  a  ray  of  star-light  is  decomposed,  their  number  and  arrangement 

effects  of  sun-light.  675.  Do  the  seven  primary  colors  possess  these  three  properties 
in  equal  degrees  ?  To  which  does  brightness  particularly  belong  ?  To  which,  heat? 
To  which,  actinism?  What  follows  from  the  peculiar  brightness  of  yellow?  How 
is  it  proved  that  the  red  portion  of  the  spectrum  has  the  most  heat  ?  How  does  the 
refraction  of  solar  heat  compare  with  that  of  solar  light  ?  Prove  this.  How  may  it 
be  shown  that  actinism  is  strongest  in  violet  and  indigo  rays  ?  676.  Describe  the  dark 
Une»  in  the  spectrum.  What  is  said  of  the  lines  found  in  spectra  produced  from  star* 


ACHROMATIC   LENSES.  259 

are  different,  nor  do  they  correspond  in  spectra  formed  by  rays  from  different 
stars.  When  rays  produced  by  electricity  or  combustion  are  decomposed 
with  the  prism,  bright  lines  are  found  crossing  the  spectrum  instead  of  dark 
ones. 

677.  DISPERSION  OF  LIGHT. — By  the  Dispersion  of  light 
is  meant  the  formation  of  a  spectrum  from  a  single  ray. 
Spectra  formed  by  different  refractive  media  are  of  differ- 
ent lengths.     Thus  flint-glass  forms  a  spectrum  about  twice 
as  long  as  crown-glass  forms,  and  four  times  as  long  as  wa- 
ter.    Flint-glass  is  therefore  said  to  have  twice  the  disper- 
sive power  of  crown-glass,  and  four  times  that  of  water. 

678.  ACHROMATIC  LENSES. — Lenses,  like  prisms,  refract 
light,  and  produce  spectra.     Rays  passing  through  a  con- 
vex lens,  therefore,  instead  of  coming  to  a  focus  at  a  single 
point,  are  more  or  less  dispersed,  and  form  colored  fringes 
about  the  focus.     This  defect  is  called  Chromatic  Aberra- 
tion.    It  was  long  a  serious  drawback  in  the  use  of  optical 
instruments ;  but  the  difficulty  is  now  remedied  by  com- 
bining two  lenses  of  such  different  materials  that  the  dis- 
persive power  of  the  one  may  nullify  that  of  the  other. 
Lenses  combined  on  this  principle  are  called  Achromatic 
Lenses. 

Achromatic  means  colorless,  and  the  lenses  are  so  called  because  they  do 
not  fringe  their  images  with  the  colors  of  the  spectrum.  A  double  convex 
lens  of  crown  glass  may  be  united  with  a  plano-concave  lens  of  flint  glass. 
The  latter  corrects  the  chromatic  aberration  of  the  former,  without  entirely 
Nullifying  its  converging  effect. 

679.  THE  RAINBOW. — The  Rainbow  is  an  arch  composed 
of  the  seven  primary  colors,  which  is  visible  in  the  sky 
when  the  sun  shines  during  a  shower.     It  appears  in  the 
opposite  quarter  to  the  sun, — in  the  west  in  the  morning, 
and  the  east  in  the  afternoon. 

When  the  sun  is  in  the  horizon,  the  rainbow  is  a  circle ;  but  the  lower 
part  of  it  is  intercepted  by  the  earth's  surface,  and  therefore  we  do  not  gen- 

^ght  ?  In  spectra  produced  from  the  light  of  electricity  or  combustion  ?  677.  What 
Is  meant  by  the  Dispersion  of  light  ?  When  are  different  media  said  to  differ  in  dis- 
persive power  ?  678.  What  is  Chromatic  Aberration  ?  How  is  it  corrected  ?  What 
does  achromatic  mean  ?  Why  are  achromatic  lenses  so  called  ?  How  may  an  achro- 
matic lens  be  formed  ?  679.  What  is  the  Kainbow  ?  Where  is  it  seen  ?  What  is  th» 


260  OPTICS. 

e  rally  see  more  than  a  semi-circle.    From  the  mast-head  of  a  ressel  or  the 
top  of  a  mountain,  more  than  a  semi-circle  is  visible. 

680.  The  rainbow  is  caused  by  the  refraction  and  reflection  of  the  sun's 
rays  by  drops  of  falling  rain.  Each  drop  operates  like  a  prism,  decomposing 
the  light  that  strikes  it.  The  observer's  eye  is  so  placed  as  to  receive  but 
one  of  the  colors  from  one  drop,  but  from  other  drops  it  receives  the  other 
colors,  and  thus  has  an  arched  spectrum  formed  complete.  As  no  two  per- 
sons occupy  exactly  the  same  spot,  no  two  can  see  exactly  the  same  bow. 

681.  Sometimes  two  distinct  bows  are  visible,  one  with- 
in the  other.  The  inner  one,  which  is  called  the  Primary 
Bow,  is  the  brighter  of  the  two.  The  outer  one  is  called 
the  Secondary  Bow;  the  rays  that  form  it  undergo  one 
more  reflection  within  the  drop  than  those  that  form  the 
primary  bow,  and  are  therefore  fainter.  In  the  primary 
bow,  the  arrangement  of  the  colors  is  the  same  as  in  the 
solar  spectrum ;  in  the  secondary  bow,  this  order  is  re- 
versed. 

682.  Whenever  the  air  is  filled  with  drops,  and  the  sun  -shines  on  them  at 
a  certain  angle,  rainbows  are  formed,  which  are  visible  to  an  observer  in  a 
proper  position.    Hence  they  are  often  seen  in  the  spray  of  water- falls  and 
fountains. 

683.  Bows  are  sometimes  similarly  formed  by  moon-light,  but  they  are 
faint  and  rarely  seen.    "When  so  formed,  they  are  called  Lunar  Rainbows. 

684.  HALOES. — Haloes  are  luminous  or  colored  circles 
seen  around  the  sun  and  moon  under  certain  conditions  of 
the  atmosphere.  They  are  more  frequently  seen  around 
the  moon,  because  the  sun's  light  is  so  intense  that  they  are 
lost  in  its  superior  brightness.  Haloes  arise  from  the  refrac- 
tion and  dispersion  of  light  by  small  crystals  of  ice  floating 
in  the  higher  regions  of  the  atmosphere. 


Vision. 

685.  THE  EYE. — The  eye  is  the  organ  with  which  we 
see.     Nothing  more  strikingly  displays  the  wisdom  of  the 

form  of  the  rainbow  ?  680.  Explain  the  principle  on  which  the  rainbow  is  formed. 
681.  When  two  bows  are  formed,  what  is  each  called,  and  which  is  the  brighter  ?  In 
what  order  are  the  colors  arranged  in  the  rainbow  ?  682.  By  what  besides  rain  may 
bows  be  produced?  683.  What  are  Lunar  Kainbows?  What  is  said  of  them? 
684.  What  are  Haloes  T  Where  are  they  most  frequently  seen  ?  How  are  haloes  pro- 


THE  EYE. 


261 


Creator  than  the  nice  adaptation  of  this  wonderful  instru- 
ment to  the  purposes  for  which  it  is  designed. 

686.  Parts  of  the  Eye. — The  human  eye  is  a  spheroid, 
about  an  inch  in  diameter,  resting  in  a  cavity  below  the 
forehead,  capable  of  being  moved  upward,  downward,  or 
sidewise,  by  muscles  attached  to  it  behind.  It  consists  of 
ten  parts : — 

6.  The  Vitreous  Humor. 

7.  The  Ret'-i-na. 

8.  The  Choroid  Coat. 

9.  The  Sclerotic  Coat. 
10.  The  Optic  Nerve. 


Fig.  253. 


1.  The  Cornea. 

2.  The  Iris. 

3.  The  Pupil. 

4.  The  Aqueous  Humor. 

5.  The  Crystalline  Lens. 

687.  When  we  look  at  an  eye  as  set  in  the  head  (see 
Figure  253),  we  see  but  three  of  these  parts :  the  Cornea 
(g) ;  the  Iris  (i) ;  and  the  Pupil  (b). 
The  Cornea  is  a  transparent  coat,  cov- 
ering the  whole  front  of  the  eye,  and 
more  convex  than  the  rest  of  the  ball. 
The  Iris  is  the  circular  membrane  in  the 
middle  of  the  cornea,  according  to  the  color  of  which  we 
say  that  the  eye  is  blue  or  black,  hazel  or  gray.  The  Pupil 
is  a  circular  opening  in  the  iris,  through  which  light  passes 
into  the  interior  of  the  eye.  Fig.  254  Fig.  254. 

represents  a  section  of  the  eye.  A  A  A 
is  the  cornea.  1 1  is  the  iris,  and  the 
opening  in  the  centre  is  the  pupil.  In 
the  following  description  reference  is 
made  to  this  Figure. 

On  passing  through  the  cornea,  a  ray  of  light 
enters  the  narrow  apartment  E,  between  the  cor- 
nea on  one  side  and  the  iris  and  crystalline  lens  on  the  other.  This  is  filled 
with  a  transparent  liquid  resembling  water,  and  called  the  Aqueous  Humor. 
Traversing  this,  the  ray  next  enters  a  transparent  body,  L,  called  from  its 
shape  the  Crystalline  Lens.  Behind  this  is  the  Vitreous  Humor,  D,  a  trans- 
duced? 685.  What  is  the  eye?  686.  Describe  the  eye.  Of  how  many  parts  does  it 
consist  ?  Name  them.  687.  Which  of  these  parts  do  we  see  when  we  look  at  an  eye 
as  set  in  the  head  ?  What  is  the  Cornea  ?  What  is  the  Iris  ?  What  is  the  Pupil  ? 
"With  the  aid  of  Fig.  254,  name  and  describe  the  various  parts  of  the  eye.  By  what  is 


262  OPTICS. 

parent  fluid  which  fills  the  greater  part  of  the  globe  of  the  eye.  This  humor 
is  enclosed  within  the  Retina,  C  C  C,  a  delicate  fibrous  membrane  resembling 
net-work,  formed  by  the  expansion  of  the  optic  nerve,  on  which  every  image 
seen  by  the  eye  is  formed.  The  Optic  Nerve,  0,  passes  through  the  back  of 
the  eye  to  the  brain,  and  conveys  to  that  organ  the  impressions  made  on  the 
retina. 

The  retina  is  surrounded  by  another  coat  called  the  Choroid,  represented 
in  the  Figure  by  a  dotted  line.  The  choroid  coat  is  lined  on  its  inner  surface 
with  black  coloring  matter,  to  prevent  any  reflection  of  light  from  the  inte- 
rior of  the  eye.  Outside  of  all  is  the  Sclerotic  Coat,  B  B  B,  a  strong  mem- 
brane, to  which  the  muscles  that  move  the  eye  are  attached.  It  envelopes  the 
whole  ball  except  the  portion  in  front  covered  by  the  cornea,  which  fits  into 
it  just  as  the  crystal  of  a  watch  fits  into  the  case. 

688.  Uses  of  the  Different  Parts. — The  outer  coats  of 
the  eye  protect  the  delicate  parts  within.  The  cornea  re- 
flects some  of  the  light  that  falls  on  it,  and  this  gives  the 
eye  its  brilliancy.  It  transmits  the  greater  part,  however, 
and  unites  with  the  aqueous  humor,  the  crystalline  lens,  and 
the  vitreous  humor,  in  bringing  the  incident  rays  to  a  focus 
and  forming  an  image  on  the  retina. 

The  iris  intuitively  regulates  the  supply  of  light  admit- 
ted into  the  eye,  contracting  and  thus  enlarging  the  pupil 
in  a  faint  light,  expanding  and  thus  diminishing  it  in  a 
strong  one.  These  changes  are  not  instantly  made.  Hence, 
when  we  pass  from  a  bright  light  into  a  room  partially 
darkened,  we  can  hardly  discern  anything  till  the  pupil  en- 
larges, so  that  more  rays  are  admitted.  When  we  go  from 
a  dark  room  into  a  bright  light,  the  eye  is  pained,  because 
the  pupil,  which  had  expanded  to  the  utmost  to  accommo- 
date itself  to  the  faint  light,  does  not  immediately  contract, 
and  more  light  is  admitted  than  the  sensitive  membrane 
can  endure. 

The  pupils  of  cats,  tigers,  and  animals  generally  that  prowl  at  night  for 
prey,  are  capable  of  being  expanded  to  such  a  degree  as  to  admit  one  hun- 
dred times  as  much  light  as  when  they  are  most  contracted.  They  can  there- 
fore see  as  well  by  night  as  by  day.  The  owl's  pupil  is  exceedingly  large  ; 

the  retina  surrounded ?  With  what  is  the  choroid  coat  lined?  "What  is  outside  ot 
all  ?  What  are  attached  to  the  sclerotic  coat  ?  6S8.  What  is  the  use  of  the  outer 
coats  of  the  eye?  Of  the  cornea?  Which  parts  unite  with  the  cornea  in  bringing 
incident  rays  to  a  focus  ?  What  is  the  use  of  the  iris  ?  Give  some  familiar  proofs  that 
the  iris  accommodates  itself  to  the  intensity  of  the  light.  What  is  said  of  the  pupil 


DEFECTS   OF  VISION.  263 

In  the  day-time,  even  when  contracted  to  the  utmost,  it  admits  so  much  light 
that  the  bird  is  nearly  blinded,  and  has  to  remain  stupidly  on  its  roost. 

689.  DEFECTS  OF  VISION. — In  a  perfect  eye,  the  rays 
that  enter  are  brought  to  a  focus  on  the  retina,  and  an  im- 
age is  there  formed.  If  the  rays  are  not  brought  to  a  focus 
by  the  tune  they  reach  the  retina,  or  come  to  a  focus  before 
reaching  it,  no  impression  is  made  on  the  optic  nerve  or 
communicated  to  the  brain,  and  consequently  no  image 
is  seen. 

Hence  arise  two  defects  of  vision.  When  the  cornea  is 
too  convex,  distant  objects  form  images  in  front  of  the  ret- 
ina, and  are  not  seen ;  only  such  objects  as  are  very  near 
the  eye  are  visible,  and  hence  persons  with  this  defect  of 
vision  are  called  near-sighted.  When,  on  the  contrary,  the 
cornea  is  not  convex  enough,  the  rays  are  not  brought  to 
a  focus  by  the  time  they  reach  the  retina,  and  no  image  is 
seen.  The  eyes  of  old  people  generally  labor  under  this 
defect,  in  consequence  of  the  waste  of  a  portion  of  the  vit- 
reous and  the  aqueous  humor,  so  that  the  crystalline  lens 
and  the  cornea  fall  in.  This  falling  in  is  just  what  the  near- 
sighted person  needs ;  accordingly  it  is  often  found  that 
those  who  are  near-sighted  in  youth  see  perfectly  well  when 
they  grow  old. 

690.  The  two  defects  of  vision  mentioned  above  are  remedied  by  the  use 
of  spectacles,  which  consist  of  lenses  of  different  shapes  placed  in  frames  be- 
fore the  eyes.    A  near-sighted  person  uses  glasses  just  concave  enough  to 
nullify  the  too  great  convexity  of  his  eye.    An  old  person  uses  glasses  with 
sufficient  convexity  to  make  up  the  deficiency  of  his  eye  in  that  respect. 

691.  Spectacles  were  first  used  about  the  end  of  the  thirteenth  century. 
It  is  supposed  that  the  world  is  indebted  to  Roger  Bacon  for  their  invention. 
Before  that  time  all  near-sighted  and  most  aged  persons  had  to  remain  in  a 
state  of  comparative  blindness. 

692.  Though  all  other  parts  of  the  eye  be  perfect,  if  the  optic  nerve  does 
not  perform  its  functions,  blindness  is  the  result.    Images  are  formed  on  the 
retina,  but  there  is  no  communication  with  the  brain,  and  no  impression 

of  beasts  that  prowl  at  night  ?  What  is  said  of  the  owl's  pupil  ?  689.  Where  are  im- 
ages formed  in  a  perfect  eye  ?  What  will  prevent  an  image  from  being  seen  ?  De- 
scribe the  two  defects  of  vision  arising  from  images'  not  being  formed  on  the  retina. 
690.  How  are  these  two  defects  of  vision  remedied  ?  What  sort  of  glasses  does  a  near- 
sighted person  use  ?  An  old  person?  691.  When  were  spectacles  first  used?  By 
whom  are  they  supposed  to  have  been  invented  ?  692.  If  the  optic  nerve  does  not 


264 


OPTICS. 


Is  produced.  For  amaurosis,  or  paralysis  of  the  optic  nerve,  there  is  no 
remedy. 

693.  IMAGES  FORMED  ON  THE  RETINA. — Images  are 
formed  on  the  retina,  just  as  in  a  dark  room,  by  light  ad- 
mitted through  an  aperture  (see  Fig.  235).  In  the  latter 
case,  as  we  have  already,  seen,  the  image  is  inverted,  and  it 
follows  that  images  formed  on  the  retina  must  be  inverted 
also.  Why  then  do  we  see  them  in  their  natural  position  ? 
This  question  it  is  hard  to  answer.  The  explanation  com- 
monly given  is  this : — That  we  see  all  things  inverted,  and 
have  always  done  so  ;  but,  inasmuch  as  we  know  by  expe- 
rience that  they  are  erect,  the  mind  of  itself,  insensibly  to 
us,  corrects  the  delusion  that  the  inversion  would  other- 
wise produce.  We  have  no  means  of  comparison ;  we  see 
nothing  erect,  to  serve  as  a  standard  and  prove  the  general 
inversion. 

694.  Another  question,  is  sometimes  asked  : — Since  we  have  two  eyes,  and 
two  images  are  formed,  one  on  each  retina,  why  do  we  not  see  two  images  of 
every  object  ?  The  answer  is,  because  both  eyes  are  inclined  to  any  given 
object  at  nearly  the  same  angle.  The  images  produced  on  the  retinas  are  very 
nearly  the  same.  The  impressions  transmitted  to  the  brain  by  the  two  branches 
of  the  optic  nerve  are  identical  and  simultaneous,  and  but  one  perception  is 
the  result.  If  we  press  on  one  of  our  eyes,  so  as  to  incline  it  towards  an  ob- 
ject at  a  different  angle  from  the  other,  we  see  two  images.  Drunken  men 
often  see  double,  because  they  lose  control  of  the  muscles  of  the  eye,  and  do 
not  direct  both  eyes  towards  a  given  object  at  the  same  angle. 

695.  VISUAL  ANGLE. — The  visual  angle  is  the  angle 
formed  by  two  lines  drawn  frojn  the  eye  to  the  extremities 

of  a  given  object. 
In  Fig.  255,  the  vis- 
ual angle  of  the  ar- 
rowBA  is  BE  A; 
that  of  the  arrow 
CDisCED. 
A  given  object 

pel-form  its  functions,  what  is  the  consequence  ?  693.  What  kind  of  an  image  is 
form'ed  on  the  retina,  and  why  ?  Since  an  inverted  image  is  formed  on  the  retina, 
why  do  we  see  objects  in  an  erect  position?  694.  Since  we  have  two  eyes,  why  do 
we  not  see  two  images  of  every  object?  How  may  we  make  two  images  visible  ? 
Why  do  drunken  men  often  see  double  ?  695.  What  is  the  Visual  Angle  ?  Show  th« 


THE  VISUAL  ANGLE.  265 

looks  large  or  small,  according  to  the  visual  angle  that  it 
forms.  Two  equal  arrows  held  up  before  the  eye  at  differ- 
ent distances,  as  in  Fig.  255,  form  different  visual  angles, 
and  therefore  seem  to  be  of  different  size.  If  we  measure 
their  apparent  lengths  with  an  interposed  rod,  we  shall  find 
the  nearer  one  to  measure  the  distance  a  #,  the  farther  one 
only  about  half  as  much,  c  d.  A  small  object  placed  near 
the  eye  may  form  as  great  a  visual  angle  as  a  very  large 
distant  object,  and  may  therefore  entirely  hide  the  latter 
when  interposed  between  it  and  the  eye. 

Accordingly,  the  nearer  an  object  is  brought  to  the  eye,  the  larger  it  ap- 
pears to  be,  and  the  farther  it  is  removed  the  smaller  it  looks.  When  the 
visual  angle  is  less  than  1/300  of  a  degree,  an  object  becomes  invisible.  A 
bird  flying  from  us  grows  smaller  and  smaller,  till  its  visual  angle  dimin- 
ishes so  that  it  can  no  longer  be  seen,  and  we  say  that  it  has  gone  out  of  sight. 

696.  In  the  case  of  familiar  objects,  experience  prevents  us  from  being 
misled  by  their  apparent  size.  Insensibly  to  ourselves,  we  make  allowance 
for  their  distance,  of  which  we  judge  by  the  distinctness  of  their  outline  and 
by  intervening  objects.  A  man  at  work  on  a  lofty  steeple  may  not  look  more 
than  two  feet  high,  yet  we  are  in  no  danger  of  mistaking  him  for  a  dwarf.  A 
distant  tree  seems  to  be  no  higher  than  a  bush ;  but,  if  we  see  a  horse  feed- 
ing beneath  it,  we  intuitively  compare  the  two,  and  arrive  at  a  correct  idea 
of  the  tree's  size. 

A  white  object  can  be  distinguished  at  a  greater  distance  than  one  of  any 
other  color,  and  is  visible  twice  as  far  when  the  sun  shines  directly  on  it  as 
when  simply  illumined  by  ordinary  light.  An  object  is  brought  out  most 
distinctly  by  a  back-ground  which  contrasts  strikingly  with  it  in  color. 
Dark-colored  eyes,  for  the  most  part,  see  farther  than  light  ones ;  and  those 
Who  are  in  the  habit  of  looking  at  remote  objects,  like  sailors,  can  discern 
minute  bodies  at  distances  which  render  them  invisible  to  ordinary  sight. 

697.  ADAPTATION  OP  THE  EYE. — One  of  the  most  re- 
markable properties  of  the  eye  is  its  power  of  adapting 
itself  to  different  intensities  of  light  and  different  dis- 
tances. The  pupil,  by  expanding  and  contracting,  regu- 
lates in  a  measure  the  supply  of  light ;  still,  the  difference 
of  intensity  in  the  light  admitted  to  the  eye  under  different 

Visual  angles  of  the  arrows  in  Fig.  255.  On  what  does  the  apparent  size  of  an  object 
depend?  Illustrate  this  with  the  Figure.  When  does  an  object  become  invisible? 
When  is  a  bird  said  to  go  out  of  sight  f  696.  In  the  case  of  familiar  objects,  what  pre- 
vents us  from  being  misled  as  to  their  size  ?  Give  some  familiar  examples.  What 
color  must  an  object  be,  to  be  distinguished  at  the  greatest  distance  ?  How  is  an  ob- 
ject most  distinctly  brought  out  ?  What  is  said  of  dark-colored  eyes  ?  697.  What  in 
12 


266  OPTICS. 

circumstances  is  very  great.  We  can  read  by  the  light  of 
the  moon  and  by  that  of  the  sun  ;  yet  the  latter  is  800,000 
times  as  intense  as  the  former. 

698.  Again,  the  eye  adapts  itself  to  different  distances. 
If  we  look  at  a  remote  object  through  a  telescope,  we  have 
to  pull  out  the  tube  to  a  certain  length,  according  to  the 
distance,  before  we  can  see  it  to  advantage.     No  such  arti- 
ficial adjustment  is  necessary  with  the  eye.    We  look  suc- 
cessively at  objects  1,  5,  10,  and  20  'feet  off;  and  in  each 
case  the  eye  instantly  adapts  itself  to  the  distance,  and  we 
see  without  an  effort. 

699.  An  object  may  move  with  such  velocity  that  we 
can  not  see  it,  as  is  the  case  with  a  cannon-ball.     This  is 
because  the  image  formed  on  the  retina  does  not  remain 
sufficiently  long  to  produce  an  impression.    When  an  image 
is  once  formed,  it  remains  from  one-sixth  to  one-third  of  a 
second  after  the  object  has  disappeared.     Hence  a  burning 
stick  whirled  rapidly  round  seems  to  form  a  circle  of  fire, 
and  a  meteor  or  a  flash  of  lightning,  instead  of  appearing 
in  a  succession  of  luminous  points,  produces  a  continuous 
train  of  light  in  the  heavens. 

Optical  Instruments. 

700.  Several  of  the  more  important  optical  instruments 
remain  to  be  described.     They  are  for  the  most  part  com- 
binations of  the  different  lenses  and  mirrors  already  men- 
tioned. 

701.  THE  CAMERA  OBSCFKA. — We  have  seen  that,  when 
rays  from  an  object  brilliantly  illuminated  are  admitted 
through  an  aperture  into  a  dark  room,  an  inverted  image 
is  formed.     This  image  is  apt  to  be  indistinct.     We  may 
give  it  a  sharper  outline  by  placing  a  double  convex  lens  in 
the  aperture,  and  receiving  the  image  on  a  white  ground 

one  of  the  most  remarkable  properties  of  the  eye  ?  Give  an  example  of  the  differ- 
ence of  intensity  in  the  light  admitted  to  the  eye.  698.  Show  how  the  eye  adapts 
itself  to  different  distances.  699.  Why  is  it  that  an  object  moving  with  very  great 
velocity  is  not  seen  ?  When  an  image  is  once  formed,  how  long  does  it  remain  after 
the  object  has  disappeared?  Give  examples.  700.  Of  what  are  optical  instruments 
for  the  most  part  combinations  ?  T01.  What  is  meant  by  the  Camera  Obscura  ?  How 


THE   CAMERA    OBSCURA. 


267 


Fig.  256. 


at  its  focus.     Such  an  arrangement  is  called  the  Camera 
Obscura,  or  dark  chamber. 

For  practical  purposes,  the  camera  obscura  must  be 
portable.  A  close  box,  painted  black  on  the  inside,  is 
therefore  substituted  for  the  darkened  room.  This  instru- 
ment enables  the  draughtsman  to  sketch  material  objects 
or  natural  scenery  with  great  ease  and  accuracy,  and  is  in- 
dispensable to  the  daguerreotypist  and  photographer. 

702.  Draughtsman's  Camera. — Fig.  256 
represents  the  camera  as  used  by  draughts- 
men. To  be  conveniently  traced,  the  image 
must  be  thrown  on  a  horizontal  surface,  and 
this  is  effected  by  making  the  opening  in  the 
top  of  the  box  and  receiving  the  rays  on  a 
mirror,  A,  inclined  at  an  angle  of  forty-five 
degrees.  From  this  mirror  they  are  reflect- 
ed to  a  meniscus,  B,  which  crosses  the  aper- 
ture, and  are  by  it  refracted  to  the  horizontal 
surface,  C  D,  where,  on  white  paper  placed  to 
receive  it,  is  formed  a  distinct  image,  which 
can  be  readily  traced  with  a  pencil.  The  up- 
per part  of  the  draughtsman's  person  is  ad- 
mitted through  an  opening  in  the  side  of  the 
box,  over  which  a  dark  curtain  must  be 
drawn,  so  as  to  exclude  all  light  except  what  enters  from  above. 

Fig.  257.  703.   Daguerreotypist' s    Camera.— As  used  in 

the  process  of  taking  daguerreotypes  and  photo- 
graphs, the  camera  has  the  form  shown  in  Fig. 
257.    A  is  a  brass  sliding  tube, 
containing  two  achromatic  dou- 
ble con  vex  lenses,  which  is  drawn 
out  far  enough  to 
bring  the  focus  at 
the  right  spot.  The 
image  is  received 
on     a    piece    of 
ground  glass,  fit- 
ted into  a  frame, 
which  slides  in  a 
groove  in  the  back 
of     the     camera. 
DAGUERREOTYPIST'S  CAMERA.  When  a  daguerre- 

is  the  camera  made  portable  ?    By  whom  is  the  camera  used  ?    702.  Describe  the 
draughtsman's  camera.    703.  Describe  the  daguerreotypisfs  camera.   How  is  the  plat* 


DRAUGHTSMAN'S  CAMERA. 


268  OPTICS. 

otype  is  to  be  taken,  the  ground  glass  is  withdrawn,  and  another  frame,  C, 
containing  a  prepared  plate,  carefully  shielded  from  the  light,  is  introduced 
in  its  place.  A  door  in  front  of  C  is  then  raised,  and  the  image  formed  by 
the  lenses  is  thus  allowed  to  fall  on  the  plate. 

The  plate  is  of  copper,  covered  on  one  side  with  a  thin  sheet  of  silver, 
which  is  rendered  sensitive  by  exposure  to  the  vapor  of  iodine.  The  rays 
transmitted  through  the  camera,  by  that  property  inherent  in  them  which 
we  have  called  actinism,  in  a  few  seconds  produce  a  chemical  effect  on  the 
sensitive  surface,  and  the  plate  is  then  removed  to  a  dark  room.  No  change 
is  visible  on  its  surface ;  but,  as  soon  as  it  is  exposed  to  the  vapor  of  mercu- 
ry, the  picture  begins  to  appear  and  soon  becomes  distinct.  It  is  produced 
by  the  adhesion  of  small  globules  of  mercury  to  those  parts  of  the  plate  that 
have  been  affected  by  light,  to  the  exclusion  of  the  rest ;  and  this  adhesion  is 
owing  to  some  chemical  change  in  the  parts  so  affected.  After  being  washed 
in  a  weak  solution  of  hyposulphite  of  soda,  and  then  in  water,  the  plate  is 
allowed  to  dry,  and  the  image  is  fixed. 

The  photographic  process  is  similar,  except  that  the  image  is  received  on 
paper  rendered  sensitive  by  different  preparations,  instead  of  on  a  metallic 
plate. 

704.  THE  MICROSCOPE. — The  Microscope  is  an  instru- 
ment which  enables  us  to  see  objects  too  small  to  be  dis- 
cerned by  the  naked  eye.     This  is  the  case  with  objects 
whose  visual  angle  is  less  than  ^7  of  one  degree  ;  the  mi- 
croscope enables  us  to  see  them  by  increasing  their  visual 
angle. 

Microscopes  are  divided  into  two  classes,  Single  and 
Compound.  A  Single  Microscope  is  one  through  which 
the  object  is  viewed  directly.  With  the  Compound  Mi- 
croscope a  magnified  image  of  the  object  is  viewed,  instead 
of  the  object  itself. 

705.  The  Single  Microscope. — The  single  microscope 
consists  of  a  double  convex  lens  (or  sometimes  more  than 
one),  through  which  we  look  at  the  object  to  be  mag- 
nified.   The  principle  on  which  it   operates  is  shown  in 
Fig.  258. 

The  arrow  b  c  would  be  seen  by  the  naked  eye  under  the  visual  angle 
b  A  c.  When  the  lens  m  is  interposed,  the  rays  are  so  refracted  as  to  form 

prepared  ?  Give  an  account  of  the  daguerreotype  process.  How  does  the  photo- 
graphic process  differ  from  it  ?  704.  What  is  the  Microscope  ?  How  does  it  enable 
as  to  see  minute  objects  ?  Name  the  classes  into  which  microscopes  are  divided 
What  is  a  Single  Microscope  ?  What  is  a  Compound  Microscope  ?  705.  Of  what  doea 
the  single  microscope  consist  ?  With  Fig.  258,  explain  the  principle  on  which  the 


THE   MICROSCOPE. 


269 


the  visual  angle  DAE,  and  the  arrow     7,  j  Fig.  258. 

appears  to  be  of  the  size  D  E,  much 
larger  than  it  really  is.  Sometimes 
an  exceedingly  minute  object  becomes 
visible  when  brought  very  near  the 
eye,  but  in  that  position  the  rays  en- 
ter the  eye  with  such  divergency  that 
a  confused  image  is  produced.  The 
microscope  corrects  this  excessive  divergency,  and  presents  a  clear  and  mag- 
nified image. 

706.  The  Compound  Microscope. — The  compound  mi- 
croscope is  a  combination  of  two,  three,  or  four  convex 
lenses,  through  which  we  view  a  magnified  image  of  an 
object  instead  of  the  object  itself.  The  lenses  are  fixed  in 
tubes  moving  one  within  the  other,  and  suitable  apparatus 
is  provided  for  adjusting  them,  for  holding  the  object  un- 
der examination,  and  throwing  on  it  a  strong  light.  When 
but  two  lenses  are  employed,  they  are  arranged  as  repre- 
sented in  Fig.  259. 

Fig.  259. 


D  E  is  the  object,  and  B,  the  lens  nearest  to  it,  is  called  the  object-glass. 
C,  the  lens  nearest  the  eye,  is  called  the  eye-glass.  A  magnified  image  of  the 
arrow  is  formed  at  H  I  by  the  lens  B.  This  image  is  viewed  through  the 
lens  C,  and  is  thus  still  further  magnified,  being  seen  under  an  increased 
visual  angle  at  F  G.  If  the  magnifying  power  of  B  is  20,  and  that  of  C  4,  the 
image  seen  will  be  80  times  the  size  of  life. 

707.  Solar  and  Oxy -hydrogen  Microscopes. — These  mi- 
croscopes are  used  for  throwing  magnified  images  on  "a 
white  screen  in  a  darkened  room. 


single  microscope  operates.  706.  Describe  the  compound  microscope.  With  the  aid 
of  Fig.  259,  name  the  parts  and  show  the  operation  of  the  compound  microscope. 
TOT.  For  what  are  the  Solar  and  the  Oxy-hydrogen  Microscope  used  ?  Describe  tho 


270  OPTICS. 

In  the  case  of  the  Solar  Microscope,  an  aperture  is  made 
in  one  of  the  shutters.  Outside  of  this  a  mirror  is  placed, 
in  the  sun,  at  such  an  angle  as  to  reflect  the  rays  that  fall 
on  it  through  a  horizontal  tube  towards  the  object  to  be 
magnified.  They  first  fall  on  a  convex  lens,  and  then  on  a 
second,  which  brings  them  to  a  focus  on  the  object,  and 
thus  illuminates  it  brilliantly.  Another  lens,  at  the  oppo- 
site extremity  of  the  instrument,  produces  the  magnifying 
effect.  A  screen,  from  ten  to  twenty  feet  off,  receives  the 
image,  which  increases  in  size  with  the  distance.  If  the 
screen  is  too  far  removed,  the  image  becomes  faint ;  but 
so  powerful  is  the  light  concentrated  on  the  object  that  a 
very  great  magnifying  effect  may  be  produced  without  any 
lack  of  distinctness. 

In  the  Oxy-hydrogen  Microscope,  the  principle  is  the  same,  but  the  bril- 
liant light  produced  by  burning  lime  in  a  current  of  oxygen  and  hydrogen  is 
substituted  for  the  rays  of  the  sun.  Accordingly,  with  this  instrument,  the 
aperture  in  the  shutter  and  the  mirror  on  the  outside  are  unnecessary.  Fig. 
260  shows  the  operation  of  the  oxy -hydrogen  microscope. 

Fig.  260.  B    represents  an  intense 

white  light  produced  by  the 
burning  of  a  cylinder  of  lime 
in  a  current  of  oxygen  and  hy- 
drogen  combined.  This  light 
falls  on  the  reflector  A,  by 
which  it  is  thrown  back  on  the  double  convex  lens  C,  and  this  brings  it  to  a 
focus  on  the  object  D.  E  is  an  achromatic  lens,  which  throws  a  magnified 
image  on  the  screen. 

708.  The  microscope  introduces  us  to  new  worlds,  of  the  very  existence 
of  which  we  would  otherwise  have  been  ignorant.  It  reveals  to  us,  in  every 
drop  of  water  in  which  vegetable  matter  has  been  infused,  swarming  myriads 
of  moving  creatures, — miniature  eels,  infinitesimal  lobsters,  ravenous  mon- 
sters with  distended  jaws  preying  on  their  feebler  fellows, — all  endowed  with 
the  organs  of  life,  and  so  minute  that  their  little  drop  is  to  them  a  world  nearly 
as  large  as  ours  to  us.  It  shows  us  the  feeding  apparatus  of  the  flea  magni- 
fied to  frightful  dimensions,  and  his  body  arrayed  in  a  panoply  of  shining 
and  curiously  jointed  scales,  studded  at  intervals  with  long  spikes.  The 
mould  on  decaying  fruit  it  magnifies  into  bushes  with  branches  and  leaves, 

solar  microscope,  and  its  operation.  What  is  the  effect  of  removing  the  screen  to  a 
greater  distance  from  the  instrument  ?  What  light  is  employed  in  the  oxy -hydrogen 
microscope  ?  With  Fig.  260,  show  how  this  microscope  operates.  708.  What  is  said 
of  the  revelations  of  the  microscope  ?  What  difference  does  it  exhibit  between  tho 


THE  MAGIC  LANTERN. 


271 


displaying  all  the  regularity  and  beauty  of  the  vegetable  creation.  It  dis- 
closes to  us  many  striking  facts  connected  with  physiology  and  chemistry. 
It  shows  us  the  imperfection  of  the  finest  works  of  art,  when  compared  with 
those  of  nature.  The  edge  of  the  sharpest  razor,  viewed  through  a  micro- 
scope, is  full  of  notches  ;  the  point  of  a  needle  is  blunt,  and  its  surface  is  cov- 
ered with  inequalities.  The  magnified  sting  of  a  bee,  on  the  other  hand,  is 
perfectly  smooth,  regular,  and  pointed.  The  finest  thread  of  cotton,  linen, 
or  silk,  is  rough  and  jagged :  whereas  in  the  filament  of  a  spider's  web  not  the 
slightest  irregularity  can  be  detected. — In  a  word,  the  revelations  of  the  mi- 
croscope are  in  the  highest  degree  wonderful  and  interesting ;  and,  to  what- 
ever we  direct  it,  we  always  find  abundant  matter  to  reward  our  labor  and 
stimulate  us  to  further  researches. 

709.  THE  MAGIC  LANTERN. — The  Magic  Lantern  is  an 
instrument  for  throwing  on  a  screen  magnified  images  of 
transparent  objects.  It  operates  on  the  same  principle  as 
the  oxy-hydrogen  microscope,  but  for  its  illuminating  power 
has  an  ordinary  lamp  instead  of  the  intense  light  produced 
by  burning  lime. 

Fig.  261. 


THE   MAGIC   LANTERN. 

Fig.  2G1  represents  the  magic  lantern.  L  is  the  lamp.  M  N  is  the  re- 
flector, which  throws  the  light  on  the  lens  A.  This  lens  brings  it  to  a  focus 
on  the  picture,  which  is  painted  on  a  glass  slider  and  introduced  into  the 
opening  C  D.  The  lens  B  receives  the  rays  from  the  slider,  and  throws  a 
magnified  image  on  the  screen  F. 

710.  Phantasmagoria. — "When  a  powerful  light  is  used, 
and  the  tube  containing  the  magnifying  lens  or  lenses  is 
capable  of  being  drawn  out  or  pushed  in,  so  as  to  bring 
them  at  different  distances  from  the  object,  we  have  what  is 
called  a  Phantasmagoria  Lantern. 


works  of  art  and  those  of  nature  ?    709.  What  is  the  Magic  Lantern  ?    How  does  it 
differ  from  the  oxy-hydrogen  microscope?    With  Fig.  261,  describe  the  magic  Ian- 


272  OPTICS. 

To  exhibit  the  Phantasmagoria,  a  transparent  screen  is  suspended,  on  one 
side  of  which  is  the  exhibitor  with  his  lantern,  on  the  other  the  spectators. 
Having  brought  the  lantern  close  to  the  screen  and  drawn  out  the  tube  till 
the  image  (which  will  be  quite  small)  is  perfect,  the  exhibitor  walks  slowly 
back.  He  thus  gradually  increases  the  size  of  the  image,  while  he  preserves 
its  distinctness  by  pushing  in  the  tube  as  he  recedes.  The  effect  on  the 
spectators  is  startling.  The  room  being  dark,  they  can  not  see  the  screen, 
but  only  the  illuminated  image,  which,  as  it  grows  larger,  appears  to  be 
moving  towards  them ;  even  those  who  are  familiar  with  the  instrument  can 
hardly  disabuse  their  minds  of  this  impression.  When  the  exhibitor  ap- 
proaches the  screen  and  pulls  out  the  tube,  the  image  becomes  smaller  and 
appears  to  recede. 

711.  Dissolving  Views. — Dissolving  Views,   in  which 
one  picture  appears  to  melt  into  another,  are  produced  by 
two  magic  lanterns,  inclined  so  as  to  throw  their  images  on 
the  same  spot.    An  opaque  shade  is  made  to  revolve  in 
front  of  the  instruments,  in  such  a  way  as  gradually  to  in- 
tercept the  rays  from  one  and  uncover  the  tube  of  the  other. 
The  first  picture  fades,  and  a  new  one  takes,  its  place,  be- 
coming more  and  more  distinct  as  the  other  disappears. 

712.  THE  TELESCOPE. — The  Telescope  is  an  instrument 
.for  viewing  distant  objects.     It  appears  to  have  been  in- 
vented by  Metius,  a  native  of  Holland,  in  1608.     The  fol- 
lowing year,  Galileo,  hearing  of  the  new  instrument,  con- 
structed one  for  himself,  and  was  the  first  to  make  a 
practical  use  of  the  invention.     To  the  Telescope,  Astron- 
omy is  indebted  for  the  important  advances  it  has  made 
during  the  last  two  centuries. 

Telescopes  are  of  two  kinds,  Refracting  and  Reflecting. 
In  the  former,  which  were  the  first  constructed,  lenses  are 
used ;  in  the  latter,  polished  metallic  mirrors. 

713.  Refracting  Telescopes. — The  simplest  form  of  the 
telescope  is  that  devised  by  Galileo.     It  is  a  tube  contain- 
ing a  convex  object-glass  and  a  concave  eye-glass.     By  the 
former  parallel  pencils  are  made  to  converge  towards  a 
focus,  where  they  would  form  an  inverted  image  ;  but  be- 

tern.  710.  What  is  the  Phantasmagoria  Lantern  ?  How  are  the  phantasmagoria  pro- 
duced? What  is  said  of  their  effect?  Til.  What  are  Dissolving  Views?  How  are 
they  produced ?  712.  What  is  the  Telescope?  By  whom  was  it  invented?  Who 
first  made  a  practical  use  of  the  invention  ?  Name  the  two  kinds  of  telescopes. 


THE  TELESCOPE.  273 

fore  reaching  the  focus  they  fall  on  the  concave  lens,  and 
have  their  convergency  so  far  corrected  that  an  object  is 
distinctly  seen  by  an  eye  at  the  extremity  of  the  tube.  The 
Opera-glass  consists  of  two  Galilean  Telescopes  combined. 
The  night-glass  used  by  sailors  is  on  the  same  plan. 

In  the  instrument  called  the  Astronomical  Telescope,  both  object-glass  and 
eye-glass  are  convex.  The  former  produces  an  inverted  image  at  its  focus ; 
the  latter,  which  is  so  placed  that  its  focus  falls  at  the  same  spot,  refracts  the 
rays  diverging  from  this  image,  and  thus  renders  it  visible  to  the  eye.  The 
inversion  of  the  image  is  of  no  consequence  in  observing  the  heavenly  bodies ; 
but,  when  objects  on  the  earth  are  viewed,  we  want  an  erect  image,  and  there- 
fore in  the  Terrestrial  Telescope  two  additional  lenses  are  introduced  to  cor- 
rect the  inversion. 

714.  Reflecting  Telescopes. — In  Reflecting  Telescopes, 
a  speculum,  or  mirror,  takes  the  place  of  the  object-glass. 
These  instruments  appear  in  several  different  forms.  The 
principle  on  which  Herschel's  is  constructed,  will  be  under- 
stood from  Fig.  262. 

The    mirror   S  S    is  Fig  2(J2. 

placed  at  the  farthest 
extremity  of  the  tube, 
inclined  so  as  to  make 
the  rays  that  fall  upon 
it  converge  towards  the 
side  of  the  tube  in  which 
the  eye-piece  a  b  is  fixed  to  receive  them.  The  observer  at  E,  with  his  back 
towards  the  heavenly  body,  looks  through  the  eye-piece,  and  sees  the  reflect- 
ed image.  His  position  is  such  as  not  to  prevent  the  rays  from  entering  the 
open  end  of  the  tube.  The  advantage  gained  with  this  instrument  depends 
in  a  great  measure  on  the  size  of  the  mirror  j  for  all  the  rays  that  fall  on  it 
are  concentrated  and  transmitted  to  the  eye. 

715.  The  largest  telescope  ever  constructed  was  made  by  the  Earl  of  Rosse. 
The  great  mirror  is  six  feet  in  diameter,  and  weighs  four  tons.  The  tube,  at 
the  bottom  of  which  it  is  placed,  is  of  wood  hooped  with  iron.  It  is  fifty-two 
feet  long  and  seven  feet  across.  It  is  computed  that  with  this  instrument 
250,000  times  as  much  light  from  a  heavenly  body  is  collected  and  transmit- 
ted to  the  eye  as  ordinarily  reaches  it. 

713.  Describe  the  Galilean  Telescope.  Of  what  does  the  Opera-glass  consist  ?  De- 
scribe the  Astronomical  Telescope.  How  does  the  Terrestrial  Telescope  differ  from 
the  Astronomical  ?  714.  In  reflecting  telescopes,  what  takes  the  place  of  the  object- 
glass  ?  With  Fig.  262,  explain  the  principle  on  which  Herschers  Telescope  operates. 
On  what  does  the.  advantage  gained  with  this  instrument  depend  ?  715.  Describe  the 
telescope  of  the  Earl  of  Eosse.  How  great  ia  the  advantage  gained  with  it  ? 

12* 


274  OPTICS. 


EXAMPLES   FOR    PRACTICE. 

1.  (See  §  594.)  How  long  does  it  take  a  ray  from  the  moon  to  reach  the  earth, 

the  moon's  distance  being  240,000  miles  ? 

2.  The  planet  Jupiter  is  496,000,000  miles  from  the  sun.    How  long  does  it 

take  a  ray  of  light  from  the  sun  to  reach  the  planet  ? 

3.  A  ray  of  light  from  the  sun  is  about  12,326  seconds  longer  in  reaching  the 

newly  discovered  planet  Neptune  than  in  reaching  Jupiter.    About  how 
many  miles  farther  from  the  sun  is  Neptune  than  Jupiter? 

4.  (See  §  595.)  A  holds  his  book  1  foot,  and  B  holds  his  3  feet,  from  a  certain 

candle.    How  much  more  light  does  A  receive  than  B  ? 

5.  The  planet  Uranus  is  twice  as  far  from  the  sun  as  the  planet  Saturn. 

How  does  the  light  received  at  Saturn  compare  in  intensity  with  that  re- 
ceived at  Uranus  ? 

6.  (See  §  650.)  How  many  times  is  the  ordinary  heat  of  the  sun  increased  by 

a  burning  glass  with  an  area  of  10  square  inches,  the  focus  of  which  has 
an  area  of  l/10  of  a  square  inch  ? 

7.  A  convex  lens  has  a  focus  l/&  of  a  square  inch  in  area,  and  increases  the 

heat  of  ordinary  sun-light  200  times ;  what  is  the  area  of  the  lens  ? 


CHAPTER  XV.. 

ACOUSTICS. 

716.  ACOUSTICS  is  the  science  that  treats  of  sound. 

717.  NATURE  AND  ORIGIN  OF  SOUND. — Sound  is  an  im- 
pression made  on  the  organs  of  hearing  by  the  vibrations 
of  elastic  bodies,  transmitted  through  the  air  or  some  other 
medium.    These  vibrations  may  be  compared  to  the  mi- 
nute waves  which  ripple  the  surface  of  a  pond  when  a  stone 
is  thrown  in, — spreading  out  from  a  centre,  but  growing 
smaller  and  smaller  as  they  recede,  till  finally  they  are  no 
longer  perceptible.     They  are  produced  by  percussion,  or 
any  shock  which  gives  an  impulse  to  the  particles  of  the 
sounding  body.     There  is  no  sound  that  can  not  be  traced 
to  mechanical  action. 

718.  Bodies  whose  vibrations  produce  clear  and  regular 


SOUND  PRODUCED  BY   VIBRATIONS.  275 

Bounds  are  called  Sonorous.    Bell-metal,  glass,  the  head  of 
a  drum,  are  sonorous. 

719.  That  sound  is  produced  by  vibrations  is  proved  in  various  ways.  A 
person  standing  near  a  piano-forte  or  an  organ,  when  it»  is  played,  feels  a 
tremulous  motion  in  the  floor  of  the  apartment,  as  well  as  in  the  instru- 
ment itself  if  he  touches  it.  We  perceive  the  same  tremor  in  a  bell  when 
in  the  act  of  being  rung.  In  like  manner,  if  we  strike  a  tumbler  so  as  to  pro- 
duce a  sound,  and  then  touch  the  top,  we  feel  an  internal  agitation ;  and, 
when  the  vibrations  are  stopped,  as  they  are  by  contact  with  the  finger,  the 
Bound  ceases  with  them.  If  we  put  water  in  a  glass  and  produce  a  sound  by 
rubbing  the  top  with  the  finger,  the  liquid  is  agitated,  and  its  motion  contin- 
ues until  the  sound  dies  away. — Place  some  fine  sand  on  a  square  piece  of 
glass,  and,  holding  it  firmly  with  a  pair  of  pincers,  draw  a  violin-bow  along 
the  edge.  The  sand  is  put  in  motion,  and  finally  settles  on  those  parts  of 
the  glass  that  have  the  least  vibratory  movement. — If  a  tuning-fork  be  struck 
and  applied  to  the  surface  of  mercury,  minute  undulations  may  be  observed 
in  the  metal. 

That  these  vibrations  are  communicated  to  the  air  and  by  it  transmitted 
to  the  ear,  also  admits  of  easy  proof.  The  rapid  passage  of  a  heavy  cart  or 
stage  shakes  the  walls  of  a  house.  The  discharge  of  artillery  sometimes  breaks 
windows.  These  effects  are  due  to  the  vibrations  suddenly  produced  in  the 
air.  If  there  is  no  air  or  other  medium  to  transmit  the  vibrations  to  the  ear, 
no  sound  is  heard.  We  have  already  seen  (§  439)  that  a  bell  rung  in  an  ex- 
hausted receiver  can  hardly  be  heard ;  if  the  air  could  be  entirely  removed, 
it  would  be  wholly  inaudible.  Sound,  therefore,  does  not  leap  from  point  to 
point,  but  is  transmitted  by  vibrations  communicated  from  one  particle  to 
another. 

720.  All  sonorous  bodies  are  elastic,  but  all  elastic  bodies 
are  not  sonorous. 

Soft  bodies  are  generally  non-elastic,  and  consequently  not  sonorous. 
This  is  the  case  with  cotton,  for  example,  which  yields  little  or  no  sound 
.when  struck  by  a  hammer.  It  is  on  this  account  that  music  loses  much  of  its 
effect  in  rooms  with  tapestried  walls  or  curtained  windows.  Hence,  also,  a 
speaker  finds  it  more  difficult  to  make  himself  heard  in  a  crowded  room  than 
in  one  that  is  empty. 

721.  TRANSMISSION  OF  SOUND. — All  the  sounds  that  or- 


716.  "What  is  Acoustics  ?  TIT.  "What  is  Sound  ?  How  are  sound- waves  produced  ? 
To  what  is  every  sound  traceable?  T18.  "What  bodies  are  called  Sonorous?  Give 
examples.  T19.  How  is  it  proved  by  familiar  experiments  that  sound  is  produced  by 
Vibrations  ?  If  a  tuning-fork  bo  struck  and  applied  to  the  surface  of  mercury,  what 
may  be  observed  ?  How  is  it  proved  that  these  vibrations  are  communicated  to  the 
air  and  by  it  transmitted  to  the  ear?  T20.  "What  property  belongs  to  all  sonorous 
bodies  ?  "What  bodies  are,  for  the  most  part,  not  sonorous  ?  Give  examples.  "What 
follows  from  the  fact  that  soft  bodies  are  not  sonorous  ?  T21.  By  what  are  the  sounds 


276  ACOUSTICS. 

dinarily  reach  our  ears  are  transmitted  to  them  by  the  air. 
Any  material  substance,  however,  that  connects  our  organ » 
of  hearing  with  a  vibrating  body,  may  transmit  the  vibra- 
tions in  the  same  way.  Thus,  with  our  heads  immersed  in 
water,  we  can  hear  a  sound  produced  under  the  surface  at 
a  considerable  distance.  Here  water  is  the  transmitting 
medium. 

722.  Liquids  are  better  conductors  of  sound  than  aeri- 
form bodies,  and  solids  than  liquids. 

Persons  in  boats  can  converse  with  each  other  at  a  great  distance,  be- 
cause water  is  a  good  conductor  of  sound.  When  the  ear  is  applied  to  one 
end  of  a  long  stick  of  timber,  the  scratch  of  a  pin  at  the  other  end  can  be 
distinctly  heard,  owing  to  the  conducting  power  of  the  wood.  An  approaching 
locomotive  can  be  heard  at  a  great  distance  by  placing  one's  ear  on  the  rails. 
The  American  Indians  knew  by  experience  the  facility  with  which  solids 
transmit  sounds,  and  were  in  the  habit  of  applying  their  ears  to  the  earth 
when  they  suspected  the  approach  of  an  enemy,  or  wanted  a  more  distinct 
impression  of  any  sound  that  attracted  their  attention. 

723.  The  denser  air  is,  the  more  readily  it  transmits  sounds.  On  tho  tops 
of  high  mountains,  where,  as  we  have  already  learned,  the  atmosphere  is 
rare,  the  human  voice  can  be  heard  only  a  few  rods  off,  and  the  report  of  a 
musket  sounds  no  louder  than  the  snapping  of  a  whip  at  the  level  of  the  sea. 
On  the  other  hand,  the  air  in  a  diving-bell  let  down  to  the  bottom  of  the  sea, 
which  is  condensed  by  the  upward  pressure  of  the  water,  transmits  sound  so 
freely  that  those  who  descend  can  hardly  speak  to  each  other  above  their 
breath ;  conversation  in  an  ordinary  tone  would  pain  the  ear. — Frosty  air  is 
a  much  better  conductor  of  sound  than  warm  air.  In  the  polar  regions,  con- 
versation has  been  carried  on  by  two  persons  a  mile  apart. 

Still  air  of  uniform  density  transmits  sounds  more  freely  than  air  which 
is  agitated  by  variable  currents  or  contains  strata  of  different  density.  This 
is  one  reason  why  sounds  are  more  distinctly  heard  by  night  than  by  clay. 
Palling  rain  or  snow  interferes  with  the  vibrations,  and  tends  to  make  sounds" 
less  distinct ;  so,  likewise,  do  contrary  winds. 

724.  If  the  air  were  perfectly  still  and  of  uniform  densi- 
ty, sound  transmitted  through  it  would  decrease  in  loud-* 
ness  as  the  square  of  the  distance  from  the  vibrating  body 

we  ordinarily  hear,  transmitted  ?  "What  else  may  transmit  sound-waves  in  the  same 
way  ?  722.  How  do  solid,  liquid,  and  aeriform  bodies  compare,  as  conductors  of 
aound  ?  Give  a  proof  of  the  conducting  power  of  water.  Stale  some  facts  illustrating 
the  facility  -with  which  solids  conduct  sound.  723.  How  do  rare  and  dense  air  com- 
pare, as  conductors  of  sound  ?  Give  examples.  How  does  cold  air  compare  with  warm 
In  conducting  power  ?  Under  what  circumstances  does  air  transmit  sound  most  free* 
?y  ?  What  ia  the  effect  of  falling  rain  or  snow  ?  724.  If  the  air  were  perfectly  still 


VELOCITY    OF  SOUND.  277 

increased.  The  report  of  a  cannon,  for  instance,  would 
seem  only  one-fourth  as  loud  at  a  distance  of  200  feet  as  at 
a  distance  of  100  feet. 

725.  VELOCITY  OF  SOUND. — Under  ordinary  circum- 
stances, sound  is  transmitted  through  air  with  a  velocity 
of  1,1 20  feet  in  a  second,  which  is  at  the  rate  of  a  mile  in 
about  4f  seconds. 

All  sounds,  whether  loud  or  faint,  high  or  low,  are 
transmitted  by  a  given  medium  with  equal  rapidity.  Were 
it  not  so,  there  would  be  no  such  thing  as  harmony  in  mu- 
sical performances,  for  the  notes  of  the  different  instruments 
would  reach  the  ear  at  different  intervals. 

Sound,  it  will  be  observed,  travels  much  more  slowly  than  light.  The 
latter  moves  192,000  miles  while  the  former  is  going  only  1,120  feet.  The 
difference  in  their  velocities  is  perceptible  even  at  short  distances.  If  we 
look  at  a  man  splitting  wood  a  few  rods  off,  we  see  the  axe  descend  on  the 
log  some  time  before  we  hear  the  noise  of  the  blow.  So,  the  report  of  a  can- 
non is  not  heard  till  after  the  flash  is  seen, — the  interval  being  long  or  short 
according  to  its  distance. 

726.  When  the  sound  is  accompanied  with  a  flash,  knowing  the  relative 
velocity  of  sound  and  light,  we  can  calculate  very  nearly  the  distance  from 
which  it  comes.  We  have  only  to  notice  the  number  of  seconds  that  elapse 
after  the  flash  is  seen  before  the  sound  is  heard,  and  multiplying  this  by 
1,120,  we  get  the  distance  in  feet.  The  time  which  it  takes  the  light  to  trav- 
erse the  given  distance  and  reach  the  eye,  is  so  small  that  it  does  not  enter 
into  the  calculation.  For  example,  if  a  clap  of  thunder  is  heard  3  seconds 
after  the  accompanying  flash  is  seen,  the  cloud  from  which  they  proceed  is  3 
times  1,120  (or  3,360)  feet  distant.  The  sooner  the  report  follows  the  flash, 
the  nearer  the  cloud. 

727.  Water  transmits  sound  4^  times  as  rapidly  as  air  ; 
iron,  10  times;  and  different  kinds  of  wood,  from  11  to  17 
times. 

Place  the  ear  at  one  end  of  a  very  long  stick  of  timber,  and  let  some  one 
strike  the  other  end  with  a  hammer.  The  wood  conducts  the  sound  to  the 
ear  so  much  more  quickly  than  the  air  that  the  blow  is  heard  twice.  So, 

and  of  uniform  density,  what  would  be  the  law  for  the  loudness  of  a  sound  heard  at 
different  distances  ?  Give  an  example.  725.  What  is  the  velocity  of  sound  ?  How 
is  the  velocity  of  sound  affected  by  its  loudness  and  pitch  ?  What  proof  have  we  of 
this?  How  does  the  velocity  of  sound  compare  with  that  of  light?  Give  some  fa- 
miliar instances  showing  their  difference  of  velocity.  726.  When  the  sound  is  accom- 
panied with  a  flash,  how  may  we  calculate  the  distance  from  which  it  comes  ?  Give 
an  example.  727.  With  what  velocity  does  water  transmit  sound,  as  compared  with 


278  ACOUSTICS. 

when  a  bell  at  the  end  of  a  long  iron  tube  is  struck,  two  sounds  are  heard  at 
the  opposite  extremity, — the  first  conducted  by  the  iron,  the  second  by  the 
air  within  it. 

728.  DISTANCE  TO  WHICH  SOUND  is  TRANSMITTED. — So 
many  changes  are  constantly  taking  place  in  the  atmos- 
phere, in  its  temperature,  moisture,  density,  and  the  veloc- 
ity and  direction  of  its  currents,  that  no  universal  law  can 
be  laid  down  as  to  the  distance  at  which  sound  is  audible. 
The  human  voice,  when  raised  to  its  highest  pitch  and  loud- 
est tones,  may  be  heard  at  the  distance  of  an  eighth  of  a 
mile  ;  the  report  of  a  musket,  at  5  miles. 

Through  the  water,  or  in  the  atmosphere  directly  over  it,  sounds  are  trans- 
mitted to  a  great  distance.  The  ringing  of  a  bell  under  water  has  been  heard 
across  the  whole  breadth  of  Lake  Geneva,  not  less  than  nine  miles.  The 
*'  all's  well "  of  the  sentinel  at  Gibraltar  has  been  distinguished  twelve  miles 
off,  and  naval  engagements  have  been  heard  at  a  distance  of  200  miles.  An 
eruption  of  the  volcano  of  St.  Vincent  has  been  heard  at  Demerara,  340  miles 
off, — the  greatest  distance  on  record  to  which  sound  has  been  transmitted  by 
the  atmosphere. 

729.  ACOUSTIC  TUBES. — It  is  theif  dispersion  in  the  sur- 
rounding air  that  makes  sounds  finally  inaudible.     Hence, 
when  they  are  confined  within  tubes,  they  are  carried  to  a 
much  greater  distance.     The  slightest  whisper  has  been 
heard  through  an  iron  pipe  3,120  feet  (more  than  half  a 
mile)  in  length. 

This  fact  has  been  turned  to  account  in  several  ways.  The  voice  is  con- 
veyed by  speaking-tubes  from  one  part  of  a  building  to  another,  frequently 
to  a  considerable  distance  and  by  a  circuitous  route.  The  Stethoscope,  an 
instrument  for  examining  the  lungs  and  other  internal  organs,  is  an  applica- 
tion of  the  same  principle.  It  is  a  hollow  cylinder  of  wood  with  a  funnel- 
shaped  extremity,  which  is  placed  on  the  organ  to  be  examined  while  the  ear 
is  applied  to  the  other  end.  The  sounds  produced  by  the  vital  action  within 
are  thus  conveyed  to  the  ear,  and  enable  the  experienced  examiner  to  judge 
whether  the  organ  is  in  a  healthy  state. 


air  ?  Iron  ?  Wood  ?  What  experiments  prove  that  solids  conduct  sound  more  rap- 
idly than  air  ?  728.  What  makes  it  impossible  to  lay  down  a  universal  law  as  to  the 
distance  at  which  sound  is  audible  ?  How  far  may  the  human  voice  be  heard  ?  The 
report  of  a  musket  ?  What  instances  are  mentioned  showing  the  great  distance  to 
which  sound  is  transmitted  by  water  ?  What  is  the  greatest  distance  on  record  to 
which  sound  has  been  transmitted  by  the  atmosphere  ?  729,  What  makes  sounda 
finally  inaudible  ?  How  may  this  difficulty  be  in  a  measure  removed  ?  How  far  has 
a  faint  whisper  been  heard  through  a  tube  ?  How  has  this  principle  been  turned  to 


THE   SPEAKING-TRUMPET.  279 

730.  The  Speaking-trumpet.— Even  if  the  tube  is  short, 
the  more  intense  pulsation  excited  in  a  column  of  confined 
air  makes  a  given  sound  audible  at  a  much  greater  distance 
than  if  it  is  at  once  diffused  in  the  atmosphere.     This  is 
proved  by  the  Speaking-trumpet,  an  instrument  used  by 
seamen  and  others  who  wish  to  give  additional  power  to 
their  voices.    The  narrowness  of  the  tube  prevents  the  easy 
flow  of  the  air  which  the  voice  sets  in  vibration.    The  or- 
gans of  articulation,  therefore,  operate  on  it  with  concen- 
trated force,  as  they  do  on  condensed  air  ;  and,  conse- 
quently, when  the  vibrations  escape  from  the  tube,  they 
are  propelled  to  a  greater  distance.    A  loud  voice  with  a 
speaking-trumpet  20  feet  long,  can  be  heard  at  a  distance 
of  three  miles.     No  one  can  use  the  speaking-trumpet  long 
without  being  exhausted,  which   shows  that  an  unusual 
effort  has  to  be  made  with  the  voice. 

731.  INTERFERENCE  OF  SOUND. — Two  sets  of  vibrations 
of  equal  intensity,  meeting  in  such  a  way  that  the  depres- 
sions of  one  correspond  with  the  elevations  of  the  other, 
interfere,  or  neutralize  each  other,  and  an  interval  of  silence 
is  the  result. 

Cause  a  tuning-fork  to  vibrate  and  hold  it  over  a  cylindrical  glass  vessel. 
Vibrations  will  soon  be  communicated  to  the  glass,  and  a  musical  note  will 
be  heard.  Place  a  similar  glass  vessel  at  right  angles  to  the  first  and  oppo- 
site the  tuning-fork,  and  the  note  previously  heard  will  cease.  Withdraw  it, 
and  the  note  is  again  heard.  The  vibrations  of  the  first  vessel  produce  the 
sound,  but  are  neutralized  by  those  of  the  second. 

732.  REFLECTION    OF    SOUND. — Vibrations  striking  a 
plane  surface  are  reflected  from  it  (like  light  and  heat)  hi 
such  a  way  as  to  make  the  angle  of  reflection  equal  to  the 
angle  of  incidence. 

733.  Echoes. — When  a  sound  is  heard  a  second  time  by 
reflection,  after  a  certain  interval,  an  Echo  is  said  to  be 
produced.     A  sound  is  sometimes  repeated  more  than  once, 

account  ?  What  instrument  is  constructed  on  this  principle  ?  Describe  the  Stetho- 
scope, and  its  operation.  730.  By  whom  is  the  Speaking-trumpet  used  ?  Explain  the 
principle  on  which  it  operates.  How  far  has  a  loud  voice  been  heard  with  a  speaking- 
trumpet  ?  731.  What  is  meant  by  the  Interference  of  sound,  and  how  is  it  caused  ? 
Give  an  example.  732.  What  is  the  law  for  the  reflection  of  sound  ?  733.  What  is  an 


280  ACOUSTICS. 

according  to  the  number  of  reflecting  surfaces  on  which  it 
strikes.  An  echo  near  Milan  repeats  a  single  syllable  thirty 
times. 

To  be  distinctly  heard,  the  echo  must  not  reach  the  ear  till  one-ninth  of  a 
second  after  the  original  sound  has  ceased.  Otherwise  they  will  run  together 
and  form  one  continuous  bound.  Hence,  the  reflecting  surface  must  be  a 
certain  distance  from  where  the  original  sound  is  produced.  The  farther  it 
is  off,  the  longer  the  reflected  sounds  will  be  in  reaching  the  observer's  ear, 
and  the  more  syllables  will  be  repeated.  At  Woodstock,  England,  there  is 
an  echo  which  repeats  from  17  to  20  syllables  ;  in  this  case  the  reflecting  sur- 
face is  distant  about  2,300  feet.  In  mountainous  regions  echoes  are  quite 
common.  There  are  several  remarkable  ones  among  the  Alps ;  and  the 
mountaineers  contrive  to  sing  one  of  their  national  songs  in  such  time  that 
the  echo  forms  an  agreeable  accompaniment. 

In  ordinary  rooms  no  echo  is  perceived,  because  the  distance  of  the  walls 
is  so  small  that  tht  reflected  sound  is  mingled  with  the  original  one  ;  but  in 
large  halls,  unless  the  principles  of  Acoustics  are  regarded,  an  unpleasant 
echo  follows  the  speaker's  words  and  makes  them  confused  and  indistinct. 

734.  Ear-trumpets. — Ear-trumpets,  used  by  deaf  per- 
sons, concentrate  and  reflect  to  the  interior  membrane  of 
the  ear,  vibrations  that  strike  it,  and  thus  render  audible 
sounds  that  could  not  otherwise  be  heard.  The  principle 
on  which  they  operate  will  be  understood  from  Fig.  263. 

Fi"  263  ^"ne  sounds  enter  the  large  end,  and  are  united  by 

successive  reflections  at  the  small  end,  which  is  applied 
to  the  ear.  The  outer  part  of  the  ear  is  itself  of  such  a 
shape  as  to  collect  the  sound-waves  that  strike  it  and  re- 
flect them  to  the  membrane  within.  To  enable  them  to 
hear  more  distinctly,  we  often  see  people  putting  up  their 
hands  behind  their  ears,  so  as  to  form  a  concave  reflect- 

THE   EAR-TRUMPET.        .  ....  .,         i 

ing  surface ;  in  which  case,  the  hand  acts  somewhat  on 
the  principle  of  the  ear-trumpet.  Instinct  teaches  animals  to  prick  up  their 
ears  when  they  want  to  catch  a  sound  more  clearly. 

Shells  of  a  certain  shape  reflect  from  their  inner  surface  the  vibrations 
that  strike  it  from  the  external  air,  and  hence  the  peculiar  sound  that  is 
heard  when  they  are  applied  to  the  ear. 

Echo  ?  In  what  case  may  a  sound  be  repeated  more  than  once  ?  How  often  does  an 
echo  near  Milan  repeat  a  syllable  ?  What  is  essential  to  the  distinctness  of  an  echo  ? 
On  what  does  the  number  of  syllables  repeated  depend  ?  Give  an  account  of  the 
echo  at  Woodstock,  England.  Where  are  echoes  quite  common  ?  What  is  said  of 
those  in  the  Alps  ?  Why  is  there  no  echo  in  ordinary  rooms  ?  734.  How  is  it  that 
Ear- trumpets  render  audible  sounds  that  could  not  otherwise  be  heard  ?  What  is 
eaid  of  the  outer  part  of  the  ear  ?  How  is  the  hand  made  to  act  on  the  principle  of 
»  speaking-trumpet ?  Why  do  animals  prick  up  their  ears?  Explain  the  roaring  of 


WHISPERING   GALLERIES.  281 

735.  Whispering  Galleries. — Sound  reflected  from  curved 
surfaces  follows  the  same  law  as  light  and  heat.     Let  two 
large  concave  brass  mirrors  be  placed  opposite  to  each 
other,  as  shown  in  Fig.  213  ;  the  ticking  of  a  watch,  or  the 
faintest  whisper  in  the  focus  of  oi*fc  is  distinctly  heard, 
after  two  reflections,  at  the  focus  or  the  other,  though  in- 
audible at  any  other  point.     Two  persons  with  their  backs 
to  each  other  can  thus  carry  on  a  conversation,  while  those 
between  them  are  not  aware  that  anything  is  being  said. 

An  apartment  in  which  such  a  reflection  is  produced  by 
the  walls  is  called  a  Whispering  Gallery.  An  oval  form  is 
the  best  for  such  a  gallery,  because  there  are  two  points 
within,  to  either  of  which  all  the  vibrations  produced  at 
the  other  are  reflected  at  the  same  instant  from  every  point 
of  the  surrounding  walls.  The  dome  of  St.  Paul's  Church, 
London,  and  that  of  the  Capitol  at  Washington,  are  exam- 
ples of  fine  whispering  galleries. 

One  of  the  most  remarkable  structures  of  this  kind  in  ancient  times  was 
"  the  ear  of  Dionysius",  a  dungeon  so  called  from  the  tyrant  of  Syracuse,  by 
whom  it  was  constructed.  The  walls  and  roof  were  so  arranged  that  every 
sound  from  within  was  reflected  and  conveyed  to  a  neighboring  apartment, 
where  the  tyrant  could  ensconce  himself  and  hear  even  the  whispers  of  his 
unsuspecting  victims. 

736.  MUSICAL  SOUNDS. — Musical  Sounds  are  produced 
by  regular  vibrations,  uniform  in  their  duration  and  in- 
tensity. 

737.  Loudness^  Pitch,  and  Quality. — In    connection 
with  musical  sounds,  three  things  must  be  considered ;  their 
Loudness,  their  Pitch,  and  their  Quality. 

The  Loudness  of  a  musical  sound  depends  on  the  extent 
of  the  vibrations  producing  it.  The  greater  the  vibrations, 
the  louder  is  the  sound. 

The  Pitch  of  a  musical  sound  depends  on  the  rapidity 


shells.  735.  "What  law  does  sound  reflected  from  curved  surfaces  follow  ?  Illustrate 
this  law  in  the  case  of  sounds  reflected  from  two  concave  mirrors.  What  is  a  Whis- 
pering  Gallery  ?  What  is  the  best  form  for  such  a  gallery,  and  why  ?  What  build- 
ings contain  whispering  galleries?  Give  an  account  of  "the  ear  of  Dionysius". 
T36.  How  are  Musical  Sounds  produced  ?  737.  What  three  things  must  be  considered 
la  connection  with  musical  sounds  ?  On  what  does  the  Loudness  of  a  musical  sound 


282  ACOUSTICS. 

of  the  vibrations  producing  it.  The  more  rapid  the  vibra- 
tions, the  higher  is  the  pitch. 

The  slowest  vibrations  that  produce  audible  musical  sounds  follow  each 
other  at  the  rate  of  8  in  a  second,  and  a  very  low  note  is  the  result.  As  the 
vibrations  become  more  rapid  the  pitch  rises,  till  they  recur  at  the  rate  of 
24,000  in  a  second,  when  a  very  high  note  is  produced.  Beyond  this  the  vi- 
brations last  so  short  a  time  that  they  no  longer  affect  an  ordinary  ear,  and 
no  musical  sound  is  heard. 

The  Quality  of  a  musical  sound  depends  on  the  nature 
of  the  vibrating  body.  The  human  voice,  the  piano,  and 
the  flute,  may  all  produce  a  note  of  precisely  the  same 
loudness  and  pitch,  and  yet  we  readily  distinguish  them 
apart.  The  difference  lies  in  their  Quality. 

738.  All  musical  sounds  are  produced  by  the  regular 
vibrations  either  of  solids  or  confined  air.     This  gives  rise 
to  a  division  of  musical  instruments  into  two  classes : — 
Stringed  Instruments,  like  the  violin ;  and  Wind  Instru- 
ments, like  the  flute. 

739.  STRINGED  INSTRUMENTS. — The  strings  used  in  mu- 
sical instruments  are  made  of  metal  or  cat-gut.     They  are 
fastened  at  each  end,  and  are  set  in  vibration  with  the  fin- 
ger, as  in  the  case  of  the  harp, — or  by  the  stroke  of  a  ham- 
mer, as  in  the  piano, — or  by  drawing  across  them  an  instru- 
ment made  for  the  purpose,  like  the  bow  of  a  violin. 

740.  To  produce  notes  of  different  pitch,  two  strings 
must  vibrate  with  different  degrees  of  rapidity.    That  they 
may  do  so,  one  must  be  longer  than  the  other,  or  thicker, 
or  stretched  more  tightly. 

The  longer  a  string  is,  with  a  given  thickness  and  tension,  the  more 
slowly  it  vibrates  and  the  graver  its  tone. — The  thicker  a  string  is,  with 
a  given  length  and  tension,  the  more  slowly  it  vibrates  and  the  graver  its 
tone. — The  more  tightly  a  string  is  stretched,  with  a  given  length  and  thick- 
ness, the  more  rapidly  it  vibrates  and  the  more  acute  its  tone. 

depend  ?  On  what,  its  Pitch  ?  How  rapidly  do  the  vibrations  that  produce  the  low- 
est audible  musical  sounds  follow  each  other  ?  How  rapidly,  those  that  produce  the 
highest  notes  ?  On  what  does  the  Quality  of  a  musical  sound  depend  ?  Give  an  ex- 
ample of  difference  in  quality.  738.  By  what  are  all  musical  sounds  produced  ?  How 
are  musical  instruments,  then,  divided  ?  739.  Of  what  are  the  strings  used  in  mu- 
sical instruments  made  ?  How  are  they  set  in  vibration  ?  740.  How  are  two  strings 
made  to  produce  notes  of  different  pitch  ?  State  the  three  laws  relating  to  the  length, 


WIND   INSTRUMENTS.  283 

Stringed  instruments  are  tuned, — that  is,  brought  to  their  proper  pitch, — 
by  turning  pegs  to  which  the  strings  are  attached.  Changes  in  the  condition 
of  the  atmosphere  affect  the  length  and  consequently  the  tone  of  the  strings. 

741.  The  music  of  the  ./Eolian  Harp  is  produced  by  the  action  of  currents 
of  air  on  strings  which  are  stretched  between  two  small  uprights  two  or  three 
feet  apart.     The  most  pleasing  combinations  of  sounds  sometimes  proceed 
from  this  simple  instrument,  commencing  with  a  strain,  soft  and  low,  as  it 
wafted  to  the  ear  from  a  distance,  then  swelling  as  if  it  were  coming  nearer, 
while  other  notes  break  forth,  mingling  with  the  first  with  indescribable 
sweetness. 

742.  In  the  case  of  the  drum,  musical  sounds  are  produced  by  the  vibra- 
tions of  a  tense  membrane  acting  on  the  same  principle  as  strings. 

743.  WIND  INSTRUMENTS. — In  wind  instruments,  such  as 
the  flute,  the  trumpet,  &c.,  musical  sounds  are  produced 
by  the  vibrations  of  air  confined  within  tubes.  In  tubes 
of  equal  diameter,  the  pitch  of  the  note  differs  according 
to  the  length  of  the  vibrating  column ;  the  shorter  the  col- 
umn, the  higher  or  sharper  the  note. 

There  are  two  ways  of  producing  notes  of  different  pitch 
with  the  same  instrument  : — 1.  By  joining  tubes  of  dif- 
ferent length  and  diameter,  as  in  the  organ.  2.  By  having 
but  one  tube  and  providing  apertures  in  it  at  different  in- 
tervals, by  uncovering  which  the  air  is  allowed  to  escape, 
and  the  internal  vibrations  are  stopped  at  any  desired  point. 
This  is  the  arrangement  in  the  flute. 

A  wind  and  a  stringed  instrument  produce  notes  of  the  same  pitch  when 
the  column  of  air  contained  within  the  former  vibrates  with  the  same  rapid- 
ity as  the  string  which  produces  the  note  of  the  latter. 

744.  The  tubes  of  wind  instruments  may  be  open  at  both  ends,  or  closed 
at  both  ends,  or  open  at  one  end  and  closed  at  the  other.     In  the  last  case, 
the  note  produced  is  twice  as  low  as  in  either  of  the  other  cases,  the  length 
of  the  tubes  being  the  same. 

745.  Musical  notes  are  produced  with  wind  instruments  by  blowing  into 
one  end,  by  causing  a  current  of  air  to  enter  an  aperture,  or  by  making 

thickness,  and  tension  of  strings.  How  are  stringed  instruments  tuned?  What 
causes  them  to  get  out  of  tune  ?  741.  How  are  the  sounds  of  the  ^Eolian  Harp  pro- 
duced ?  Describe  the  music  of  this  instrument.  742.  How  are  musical  sounds  pro- 
duced in  the  case  of  the  drum  ?  743.  How  are  musical  sounds  produced  in  wind 
instruments  ?  On  what  does  the  pitch  of  the  note  depend  ?  How  many  ways  are 
there  of  producing  notes  of  different  pitch  with  the  same  wind  instrument?  Mention 
them.  "When  do  a  wind  and  a  stringed  instrument  produce  notes  of  the  same  pitch  ? 
744  What  is  said  respecting  the  openings  of  the  tubes  of  wind  instruments  ? 
^45.  What  three  modes  of  producing  musical  notes  with  wind  instruments  are  men- 


284  ACOUSTICS. 

such  a  current  act  on  thin  plates  of  metal  or  wood  properly  arranged 
within. 

746.  A  jet  of  hydrogen  gas,  ignited  and  made  to  pass  through  a  glass  tube 
about  an  inch  in  diameter,  produces  sweet  musical  sounds,  which  may  be 
made  soft  or  loud  at  pleasure  by  raising  or  lowering  the  tube.  These  sounds 
are  caused  by  vibrations  excited  in  the  confined  air  by  the  burning  hydrogen. 

747.  The  Organ. — The  grandest  and  most  complicated 
of  wind  instruments  is  the  organ.     It  combines  the  tones 
of  almost  every  other  wind  instrument,  in  such  a  way  that 
they  may  be  used  singly  or  together  at  the  pleasure  of  the 
performer.     An  organ  in  Switzerland  has  tones  so  closely 
resembling  those  of  the  human  voice,  that  visitors  who  hear 
it  imagine  they  are  listening  to  a  full  choir  of  singers.    The 
great  organ  at  Haarlem,  in  Holland,  which  is  the  most  cel- 
ebrated one  in  the  world,  has  no  less  than  5,000  pipes,  as 
the  tubes  of  the  organ  are  technically  called. 

The  water-organ,  or  hydraulicon,  was  known  more  than  two  hundred 
years  before  the  Christian  era.  Its  invention  is  attributed-  to  Ctesibius,  the 
barber  of  Alexandria,  already  mentioned  as  the  inventor  of  the  lifting-pump. 
Wind-organs  appear  to  have  been  little  known  until  the  eighth  century  after 
Christ,  though  perhaps  invented  some  time  before.  We  read  that  an  instru- 
ment of  this  kind  was  sent  to  King  Pepin,  of  France,  in  the  year  757,  by  the 
Greek  Emperor,  Constantino. 

748.  THE  GAMUT. — Notes  are  said  to  be  in  unison  when 
the  vibrations  that  produce  them  are  performed  in  equal 
times. 

Two  notes,  one  of  which  is  produced  by  twice  as  many 
vibrations  as  the  other,  are  called  Octaves.  In  passing 
from  a  note  to  its  octave,  there  are  several  intermediate 
sounds,  produced  by  intermediate  numbers  of  vibrations, 
each  of  which  the  ear  recognizes  as  a  distinct  note.  These 
notes  are  distinguished  by  different  names,  as  shown  be- 
low. Assuming  the  number  of  vibrations  producing  the 
first  to  be  1,  the  relative  number  of  vibrations  producing 


tioned  ?  746.  How  may  musical  notes  be  produced  with  a  jet  of  hydrogen  gas  ? 
747.  What  is  the  grandest  of  wind  instruments?  What  are  combined  in  the  organ? 
What  is  said  of  an  organ  in  Switzerland  ?  How  many  pipes  has  the  great  Haarlem 
organ?  How  long  ago  was  the  water-organ  known?  By  whom  was  it  invented? 
When  do  wind-organs  appear  to  have  first  become  known  ?  748.  When  are  notes 
kaid  to  be  in  unison  f  What  is  meant  by  Octaves  ?  Between  a  note  and  its  octave, 


THE  GAMUT.  285 

the  other  notes  will  be  expressed  by  the  fractions  respec- 
tively placed  below  them,  the  number  of  the  eighth  note 
being,  as  already  stated,  double  that  of  its  octave. 
Names  of  the  notes,    CDEFGABC 

or,  do     re      mi     fa     sol     la       si      do 

Pronounced,  do     ra     me  fah   sole   lah     se      do 

No.  of  vibrations,  1  I  f  i  v  f  -V5-  2 
These  eight  notes  constitute  the  Gamut,  or  Diatonic  Scale.  The  notes 
of  the  next  higher  octave  bear  the  same  relations  to  each  other,  but  are  pro- 
duced bj  vibrations  performed  in  half  the  time,  and  therefore  twice  as  nu- 
merous in  each  case.  The  notes  of  the  next  lower  octave  again  bear  the  same 
relations  to  each  other,  but  their  vibrations  take  twice  the  time,  and  are  there- 
fore only  half  as  numerous.  In  other  words,  a  given  note  of  any  octave  is 
produced  by  vibrations  twice  as  rapid  as  the  same  note  of  the  next  octave 
below,  and  only  half  as  rapid  as  the  same  note  of  the  next  octave  above. 

749.  HARMONY. — Some  notes,  reaching  the  ear  simul- 
taneously, produce  an  agreeable  impression  in  consequence 
of  their  vibrations'  frequently  coinciding,  and  constitute 
what  is  called  concord.     Other  notes,  whose  vibrations 
rarely  coincide,  impress  the  ear  unpleasantly  and  produce 
discord.    A  combination  of  concordant  musical  sounds  is 
called  a  Chord.    An  agreeable  succession  of  musical  sounds 
constitutes  Melody.     A  succession  of  chords  constitutes 
Harmony. 

The  most  agreeable  concord  is  that  of  the  octave  ;  next, 
the  fifth  ;  then,  the  fourth  ;  and  then,  the  third.  Thus,  in 
the  scale  given  above,  concord  is  produced  when  C  is  sound- 
ed with  its  octave  C,  and  with  the  notes  G,  F,  and  E. 

750.  THE  HUMAN  VOICE. — The  sounds  of  the  human 
voice,  whether  used  in  speaking  or  singing,  are  produced 
by  the  vibrations  of  two  membranes  stretched  across  a 
tube,  which  connects  the  mouth  with  the  lungs.     This  tube 
is  the  wind-pipe ;  and  the  upper  part  of  it,  which  consists 

what  occur  ?  Name  the  notes  by  letters.  Give  their  other  names.  Assuming  the 
number  of  vibrations  that  produce  C  to  be  1,  mention  the  relative  numbers  that  pro- 
duce the  other  notes.  What  do  these  eight  notes  constitute  ?  What  relation  do  the 
notes  of  the  next  higher  octave  bear  to  these  ?  The  notes  of  the  next  lower  octave  ? 
749.  What  is  meant  by  Concord  ?  By  Discord  ?  What  is  a  Chord  ?  What  is  Melo- 
dy? What  is  Harmony?  Which  is  the  most  agreeable  concord?  Which  next? 
Which  next  ?  750.  How  are  the  sounds  of  the  human  voice  produced  ?  Describe 


286 


ACOUSTICS. 


Fig.  264. 


of  cartilage,  is  called  the  Larynx.  The  larynx  is  flattened 
at  the  top,  and  terminates  in  two  membranes,  which  nearly 
close  the  passage,  leaving  between  them  a  narrow  opening, 
known  as  the  Glottis.  These  two  membranes  are  called 
the  Vocal  Chords,  and  it  is  by  their  vibration,  caused  by 
the  passage  of  the  air  breathed  out  from  the  lungs,  that  the 
sounds  of  the  voice  are  produced.  Small  muscles  enable 
us  to  stretch  the  vocal  chords  more  or  less  tightly  at  pleas- 
ure, and  also  to  enlarge  or  diminish  the  opening  between 
them.  By  these  means  we  produce  notes  of  different  pitch. 
To  produce  a  change  of  note,  we  have  only  to  make  a  dif- 
ference of  yaVo-  of  an  inch  in  the  length  of  the  vocal  chords. 
Fig.  264  represents  the  glottis  under  differ- 
ent circumstances.  The  upper  plate  shows  it 
at  rest :  b,  b,  represents  the  top  of  the  larynx, 
and  c,  c,  the  vocal  chords,  relaxed  so  that  the 
breath  passing  through  the  opening  makes  no 
sound.  The  lower  plate  shows  the  glottis  in  the 
act  of  emitting  a  musical  sound,  the  chords  be- 
ing now  tightly  stretched,  and  made  to  vibrate 
by  the  air  breathed  out  between  them,  o  is  a 
passage  leading  into  the  wind-pipe,  which  re- 
mains open,  however  close  to  each  other  the 
chords  may  be  brought. 

751.  The  vocal  chords  are  shorter  in  boys 
and  women  than  in  men  ;  hence  the  voices  of 
the  former  are  sharper  or  higher  than  those  of 
the  latter.  When  boys  reach  the' age  of  14  or  15, 
the  vocal  chords  rapidly  enlarge,  and  the  voice 
is  said  to  change. — The  more  forcibly  the  air  is 
expelled  from  the  lungs  through  the  wind-pipe  and  larynx,  the  louder  is  the 
voice. 

752.  His  surprising  flexibility  of  voice  enables  man  to  imitate  almost  ex- 
actly, not  only  the  cries  of  birds  and  beasts,  but  also  the  sounds  of  various 
musical  instruments.  This  was  shown  by  the  performances  of  a  band  of 
twelve  Germans  a  short  time  since  in  the  principal  cities.  Each  imitated  a 
different  instrument  with  his  voice,  and  so  accurately,  that  those  who  heard 

the  Larynx  and  the  Glottis.  "What  are  the  membranes  stretched  across  the  top  of 
the  larynx  called  ?  How  do  we  produce  notes  of  different  pitch  ?  How  great  a  dif- 
ference in  the  length  of  the  vocal  chords  produces  a  change  of  note  ?  Point  out  the 
different  parts  in  Fig.  264.  751.  Why  are  the  voices  of  men  deeper  than  those  of 
boys  and  women  ?  What  causes  the  voices  of  boys  to  change  f  On  what  does  the 
loudness  of  the  voice  depend?  752.  What  is  said  of  the  flexibility  of  the  humaa 


THE  GLOTTIS   AND  YOCAL 
CHORDS. 


THE  HUMAN  VOICE.  287 

them  could  hardly  believe  they  were  not    listening    to  an   instrumental 
concert. 

753.  Ventriloquism. — Some  persons  have  the  faculty  of 
uttering   sounds   and   words   without   moving   their   lips. 
When,  besides  this,  they  can  throw  their  voice  into  any 
object  (as  the  expression  is),  or  make  it  seem  to  come  from 
a  distance,   they  are  called  Ventriloquists.     By  practice 
ventriloquists  attain  to  wonderful  power  over  their  voices. 

Amusing  exhibitions  of  ventriloquism  are  often  given,  in  which  the  per- 
former imitates  to  perfection  the  buzzing  of  bees,  the  grunting  of  pigs,  tha 
spitting  of  cats,  the  chirping  of  crickets,  the  drawing  of  corks,  the  gurgling 
of  liquids,  the  moaning  of  the  wind,  the  puffing  of  a  locomotive,  the  cry  of  a 
young  infant,  conversation  between  different  parties  represented  as  approach- 
ing or  receding,  in  different  parts  of  the  room,  under  tables,  &c. — It  is  sup- 
posed that  the  priests  of  the  ancient  oracles  practised  ventriloquism,  and 
thus  made  their  responses  appear  to  come  from  shrines,  statues,  &c. 

754.  Stammering. — Stammering  is  a  defect  in  speech 
caused  by  the  organs'  not  performing  their  respective  parts 
in  regular  succession.     A  convulsive  nervous  action  inter- 
feres with  their  operation. 

755.  The  difficulty  in  the  case  of  deaf  mutes  does  not 
lie  in  any  imperfection  of  the  organs,  but  proceeds  simply 
from  their  deafness.     Having  never  heard  their  own  voices 
or  those  of  others,  they  are  utterly  unable  to  appreciate 
sounds  or  adjust  the  organs  properly  for  their  articulation. 

756.  VOICES  OF  THE  INFERIOR  ANIMALS. — Man  alone  has 
the  power  of  articulation.     The  inferior  animals  utter  cries 
of  different  kinds,  according  to  the  conformation  of  the  lar- 
ynx and  the  nasal  cavities  connected  with  it.     Some  of  the 
cat's  tones  very  closely  resemble  those  of  the  human  voice. 

The  sounds  of  insects  are  produced  in  various  ways, — by  the  rapid  vibra- 
tion of  their  wings,  the  rubbing  of  their  minute  horns  against  each  other, 
the  striking  of  their  organs  on  the  bodies  around  them,  &c. 

757.  THE  HUMAN  EAR. — The   human   ear  consists  of 
three  distinct  parts ;  the  outer  ear,  the  drum,  and  the  in- 
voice ?    "What  instance  of  its  remarkable  flexibility  is  given  ?    753.  What  is  Ventril- 
oquism ?    Describe  some  of  the  feats  of  ventriloquists.    What  use  is  supposed  to 
have  been  made  of  ventriloquism  in  ancient  times  ?    754.  What  is  the  cause  of  Stam- 
mering ?    755.  Why  are  deaf  mutes  unable  to  use  their  voices  ?    756.  What  is  said 
of  the  tones  of  the  inferior  animals  ?    How  are  the  sounds  of  insects  produced  ? 


288  ACOUSTICS. 

ner  ear.  These  parts  and  their  connections  are  represented 
in  Fig.  265. 

p|    265  A  A  is  the  outer  ear,  which  acts  on 

the  principle  of  the  ear-trumpet,  collect- 
ing the  sound-waves  and  reflecting  them 
along  the  pipe  B  to  the  membrane  C, 
called  the  membrane  of  the  tympanum.  t 
E  is  the  tympanum  or  drum,  bounded  by' 
the  membrane  C  on  the  one  side,  and  the 
membrane  F  on  the  other,  and  filled  with 
air,  which  it  receives  from  the  tube  D, 
THK  HUMAN  BAB.  communicating  with  the  mouth.  G,  the 

inner  ear,  contains  a  number  of  ducts,  and  is  filled  with  a  liquid  in  which 
*ne  acoustic  nerve  floats. 

The  sound-waves  transmitted  from  the  outer  air  cause  the  membrane  C 
to  vibrate  C  excites  vibrations  in  the  air  confined  in  the  drum,  and  this  in 
turn  causes  F  to  vibrate.  The  liquid  in  the  inner  ear  receives  the  vibrations 
from  the  membrane  F,  and  transmits  them  to  the  acoustic  nerve,  by  which 
they  are  conveyed  to  the  brain,  and  the  sensation  of  hearing  is  produced. 
When  a  person  takes  cold,  the  tube  which  connects  the  drum  with  the  mouth 
is  apt  to  be  obstructed,  and  temporary  deafness  is  the  consequence. 

EXAMPLES   FOB   PRACTICE. 

1.  (See  §  724.)  If  the  air  were  perfectly  still  and  uniform  in  density,  how  would 

the  report  of  a  musket  heard  by  a  person  50  feet  off  compare  in  loudness 
with  the  same  report  heard  at  a  distance  of  250  feet? 

2.  A  cannon  is  heard  a  quarter  of  a  mile  off  with  a  certain  degree  of  loudness. 

How  far  must  a  person  be  removed,  to  hear  it  with  only  »/ioo  of  its  former 
distinctness  ? 

3.  (See  §  725.)  How  far  does  sound  travel  through  air  in  10  seconds  ?    In  20 

seconds  ?    In  one  minute  ? 

4.  How  much  faster  does  the  sound  produced  by  the  discharge  of  a  cannon 

travel,  than  that  produced  by  the  snapping  of  a  whip  ? 

5.  (See  §  726.)  I  see  the  flash  of  a  cannon  two  seconds  before  I  hear  its  re- 

port.   How  far  is  it  off? 

6.  A  clap  of  thunder  does  not  reach  the  ear  till  four  seconds  after  the  accom- 

panying flash  is  visible.    How  far  off  is  the  thunder-cloud  ? 

7.  A  thunder-cloud  is  distant  about  one  mile.  How  many  seconds  will  elapse 

between  the  flash  and  the  clap  ? 

8.  (See  §  727.)  About  how  many  feet  will  sound  travel  through  water  in  10 

seconds  ?    Through  iron  ?    Through  wood  ? 

757.  Name  the  parts  of  which  the  human  ear  consists.  With  the  aid  of  Fig.  265, 
point  out  the  different  parts,  and  show  the  operation  of  the  organ.  Why  is  tempo- 
rary deafness  produced  by  a  cold  ? 


ELECTRICITY.  289 


CHAPTER  XVI. 

ELECTRICITY. 

758.  IF  a  dry  glass  tube  or  a  stick  of  sealing-wax  be 
rubbed  with  a  piece  of  flannel,  and  then  held  a  short  dis- 
tance above  some  shreds  of  cotton,  they  will  be  instantly 
attracted  to  it,  and  after  adhering  to  its  surface  for  an  in- 
stant again  thrown  off.     A  peculiar  odor  is  perceived ;  and 
the  face,  when  brought  near  the  glass  or  wax,  feels  as  if  a 
cobweb  were  in  contact  with  it.    If  the  tube  or  sealing-wax 
be  presented  to  a  metallic  body  in  a  dark  room,  a  spark, 
accompanied  by  a  sharp  cracking  sound,  will  be  seen  dart- 
ing from  it  to  the  metal. 

The  property  thus  developed  by  friction  is  called  Elec- 
tricity. The  body  in  which  it  is  developed  is  called  an 
Electric,  and  is  said  to  be  excited  or  electrified.  The  at- 
traction exerted  by  the  excited  electric  over  light  bodies 
is  called  Electrical  Attraction.  The  substance  by  whose 
friction  the  electric  is  excited  is  known  as  the  Rubber. 

759.  ELECTRICITY  AS  KNOWN  TO  THE  •  ANCIENTS. — The 
term  electricity  is  derived  from  the  Greek  word  electron, 
amber,  the  property  in  question  having  been  first  observed 
in  that  substance. 

Thales,  one  of  the  seven  wise  men  of  Greece,  who  flourished  600  years 
B.  c.,  is  said  to  have  discovered  electricity  in  amber :  Theophrastus  and 
Pliny,  at  a  later  date,  speak  of  the  attraction  of  amber  for  leaves  and  straws. 
Both  Pliny  and  Aristotle  were  acquainted  with  the  electrical  properties  of  the 
torpedo ;  and  we  are  informed  that  a  freedman  of  the  Emperor  Tiberius  cured 
himself  of  gout  by  the  use  of  its  shocks.  Yet  the  ancients  appear  to  have 
known  nothing  more  than  a  few  isolated  facts  connected  with  the  subject ; 
and  as  a  science  Electricity  had  no  existence  till  the  commencement  of  the 
seventeenth  century. 

758.  If  a  glass  tube  or  a  stick  of  sealing-wax  be  rubbed  with  flannel,  what  phe 

nomena  will  be  observed  ?    Name  and  define  the  terms  used  in  connection  with  this 

experiment.    759.  What  is  the  derivation  of  the  term  electricity  f  What  allusions  are 

aaade  to  this  property  by  ancient  authors  ?    When  did  electricity  originate  as  a  sci- 

13 


290 


ELECTRICITY. 


760.  SOURCES  OF  ELECTRICITY.  —  Electricity  is  devel- 
oped —  1.  By  friction.  2.  By  chemical  action.  3.  By  mag- 
netism. 4.  By  heat. 


Electricity  developed  by  Friction. 

761.  Friction  is  one  of  the  commonest  sources  of  elec- 
trical excitement.  Every  one  has  noticed  how  his  hair 
crackles  under  the  comb  in  frosty  weather.  The  same  sound 
is  heard  on  stroking  the  back  of  a  cat,  and  if  the  room  is 
dark  sparks  may  be  drawn  from  its  fur. 

A  striking  example  of  the  exciting  power  of  friction  is  often  afforded  in 
factories.  The  endless  bands  by  their  friction  on  the  wheels  develop  elec- 
tricity in  great  abundance,  sometimes  yielding  sparks  at  a  distance  of  two  or 
three  feet.  In  the  carding-rooms  of  cotton  mills,  fibres  of  cotton  are  kept 
dancing  to  and  fro  by  alternate  attractions  and  repulsions,  so  that  steam  has 
to  be  let  in  from  time  to  time  to  dissipate  the  electric  fluid. 


762.    ELECTRICAL   ATTRACTION  AND  REPULSION. — "^ 
have  already  noticed  the  alternate  attraction  and  repulsion 
of  shreds  of  paper,  cotton,  and  similar  sub-        Fig.  266. 
stances  by  excited  electrics.    These  phenom- 
ena may  be  further  exhibited  with  the  appa- 
ratus represented  in  Fig.  266,  which  consists 
of  a  pith  ball  suspended  from  a  pillar  by  a 
long  silken  thread. 

Experiment  1. — Rub  a  glass  tube  with  flannel,  and  pre- 
sent it  to  the  pith  ball ;  the  latter  will  be  instantly  attract- 
ed to  the  tube.  After  they  have  remained  in  contact  an 
instant,  the  ball  will  be  thrown  off.  If  we  now  present  the 
tube  a  second  time,  the  ball,  instead  of  being  attracted, 
will  be  repelled.  After  touching  the  ball  with  the  finger, 
to  deprive  it  of  the  electricity  it  has  received  from  the 
tube,  repeat  the  experiment  with  an  excited  stick  of 
sealing-wax,  and  the  same  phenomena  will  be  exhibit- 
ed,—that  is,  the  ball  will  at  first  be  attracted, 
but  on  the  second  application  of  the  wax  will  be 
repelled.  We  find,  then,— 1.  That  both  the  

once?  760.  By  -what  is  electricity  developed?  761.  What  familiar  instances  are 
mentioned  of  the  production  of  electricity  by  friction  ?  What  striking  example  is 
afforded  in  factories  ?  762.  What  does  Fig.  266  represent  ?  What  may  it  be  used  to 
illustrate  ?  Describe  the  first  experiment  with  the  pith  ball.  What  two  facts  ar«» 


o 


ELECTRICAL  ATTRACTION  AND   REPULSION. 


291 


Fig.  267. 


glass  and  the  sealing-wax  attract  the  ball  before  they  have  communicated  to 
it  any  of  their  own  electricity.  2.  That,  after  so  doing,  they  both  repel  the 
ball. 

Experiment  2. — Suspend  two  pith  balls  from  a  pil- 
lar by  silk  threads,  and  present  to  them  an  electrified 
glass  tube  or  piece  of  sealing-wax.  They  will  both 
be  attracted;  but,  on  withdrawing  the  electric,  in- 
stead of  hanging  vertically,  they  will  repel  each-other, 
as  shown  in  Fig.  267. 

Experiment  3. — Excite  the  glass  tube,  present  it 
to  the  ball  represented  in  Fig.  266,  withdraw  it  after 
a  second  or  two,  and  then  present  the  excited  sealing- 
wax.  The  ball,  instead  of  being  repelled,  is  now  at- 
tracted. Reverse  the  experiment  by  presenting  first 
the  excited  wax  and  then  the  glass,  and  the  latter  in 
like  manner  will  be  found  to  attract  the  ball. 


763.  From  these  experiments  it  has 

been  inferred  that  there  are  two  kinds  of  electrical  excite- 
ment :  that  produced  by  glass,  which  has  been  called  Vit- 
reous or  Positive  Electricity ;  and  that  produced  by  sealing- 
wax,  which  has  been  called  Resinous  or  Negative  Electric- 
ity. We  may  lay  down  the  general  law  that  substances 
charged  with  opposite  electricities  attract  each  other,  while 
those  charged  with  like  electricities  repel  each  other. 

764.  NATURE  OP  ELECTRICITY. — What  electricity  is, — 
whether  it  is  an  imponderable  material  substance,  or  con- 
sists in  the  vibrations  of  some  subtile  medium,  or  is  simply 
a  condition  of  matter, — we  are  unable  to  say.     It  was  for- 
merly supposed  to  be  an  exceedingly  subtile  fluid  pervading 
all  things,  and  for  convenience'  sake  the  expression  electric 
fluid  is  still  retained.     The  leading  theories  respecting  the 
nature  of  electricity  are  Du  Fay's,  Franklin's,  and  Fara- 
day's. 

Du  Fay's  Theory.— Du  Fay,  a  French  philosopher,  held  that  there  are  two 
distinct  electric  fluids  (named  by  him  Vitreous  and  Resmous),  each  of  which 
attracts  the  other,  but  exhibits  repulsion  among  its  own  particles.  That  in 
their  natural  state  these  fluids  pervade  all  bodies  in  equal  quantities,  and 

shown  by  this  experiment  ?  Describe  the  second  experiment.  The  third  experi- 
ment. 763.  What  has  been  inferred  from  these  experiments  ?  What  general  law 
may  be  laid  down  ?  764.  What  is  said  of  the  nature  of  electricity  ?  What  was  it 
formerly  supposed  to  be  ?  By  what  names  are  the  leading  theories  respecting  the 
of  electricity  distinguished?  Give  the  substance  of  Du  Fay's  theory.  Of 


292  ELECTRICITY. 

combining  nullify  each  other ;  it  is  only  when  this  quiescent  compound  fluid 
is  decomposed  by  friction,  or  any  other  agency,  that  electrical  phenomena 
are  exhibited. 

Franklin's  Theory. — Dr.  Franklin,  whose  views  were  once  generally  re- 
ceived by  scientific  men,  believed  that  there  is  but  one  electric  fluid,  of  which 
every  body  in  its  natural  state  possesses  a  certain  quantity.  That  no  evi- 
dences of  the  existence  of  this  fluid  are  observed  as  long  as  a  body  retains 
its  natural  quantity ;  but,  when  it  has  either  more  or  less  than  this,  it  exhib- 
its certain  phenomena  and  is  said  to  be  electrified.  When  overcharged,  a 
body  exhibits  the  phenomena  displayed  by  glass  when  excited  by  flannel,  and 
to  such  an  electrical  condition  Franklin  applied  the  term  Positive ;  when  de- 
prived of  its  proper  share,  its  phenomena  are  the  same  as  those  of  excited 
resinous  substances,  and  such  an  electrical  state  he  called  Negative.  When 
communication  is  established  between  a  positive  and  a  negative  body,  the 
former  shares  its  superfluous  electricity  with  the  latter,  till  equilibrium  is 
established  between  them.  Du  Fay  made  the  difference  between  the  two 
electricities  to  consist  in  quality ;  Franklin,  in  quantity. 

Faraday's  Theory. — Faraday,  an  eminent  English  authority,  regards  elec- 
tricity as  simply  a  condition  of  matter.  According  to  his  theory,  an  electri- 
fied bodv  is  not  pervaded  by  any  fluid  at  all,  but  simply  endowed  with  a  cer- 
tain property  which  under  other  circumstances  it  does  not  possess. 

765.  Why  the  electricity  of  one  body  when  excited  is 
positive  and  that  of  another  negative,  we  can  not  tell. 
There  is  no  law  by  which  it  can  be  determined,  before  ex- 
periment, what  kind  of  electricity  a  body  will  exhibit.    In- 
deed, the  same  body  exhibits  different  kinds  when  rubbed 
by  different  substances.     Thus,  polished  glass  is  positively 
electrified,  when  excited  with  flannel,  but  negatively  when 
rubbed  on  the  back  of  a  cat.     Rough  glass  is  negatively 
electrified  when  rubbed  with  flannel,  but  positively  when 
excited  by  dry  oiled  silk. 

766.  Electricity  is  confined  to  the  surface  of  an  excited 
body ;  it  does  not  extend  to  the  interior.     A  hollow  ball 
may  therefore  contain  just  as  much  electricity  as  a  solid 
ball  of  the  same  size. 

767.  Positive  electricity  is  never  produced  without  neg- 
ative, or  negative  without  positive. 

Franklin's.  In  -what  did  Du  Fay  make  the  difference  between  the  two  electricities' 
to  consist  ?  In  what,  Franklin  ?  What  is  Faraday's  theory  ?  765.  Why  is  the  elec- 
tricity of  one  body  positive,  and  that  of  another  negative  1  What  is  said  of  the  elec- 
tricity of  a  body  when  rubbed  by  different  substances  ?  Give  examples.  T66.  In 
what  part  of  a  body  does  its  electricity  reside  ?  767.  By  what  is  the  production  of 


CONDUCTION   OF  ELECTRICITY.  293 

When  a  glass  tube  is  excited,  the  rubber  is  negatively  electrified ;  and 
positively,  when  sealing-wax  is  excited.  This  may  be  shown  by  applying 
the  rubber  to  a  pith  ball  charged  with  the  electricity  which  it  has  excited 
either  in  glass  or  sealing-wax.  The  ball  is  invariably  attracted,  which  shows 
that  the  electricity  of  the  rubber  is  opposite  to  that  of  the  electric  it  has  ex- 
cited. 

768.  ELECTRICS  AND  NON-ELECTRICS. — All  bodies  can  be 
electrified,  but  not  with  equal  facility.   Those  that  are  easily 
excited,  are  called  Electrics  ;  those  that  it  is  hard  to  excite, 
Non-electrics.     The  metals  generally  are  non-electrics. 

769.  CONDUCTION  OF  ELECTRICITY. — If  we  touch  the  two 
pith  balls  represented  in  Fig.  267  as  repelling  each  other 
(because  charged  with  the  same  electricity)  with  a  glass 
rod,  they  will  continue  to  repel  each  other ;  but,  if  we  touch 
them  with  a  metallic  rod,  they  will  fall  and  hang  vertically. 
This  is  because  glass  does  not  draw  off  their  electricity, 
while   metal   does.     Some   substances,  therefore,   conduct 
electricity,  while  others  do  not. 

Substances  that  transmit  electricity  freely  are   called 
Conductors  ;  those  that  do  not,  Non-conductors. 

As  a  general  thing,  the  non-electrics  are  conductors,  and  the  electrics  non- 
conductors. Some  of  the  chief  conductors  are  the  metals  (silver  and  copper 
ranking  among  the  best),  charcoal,  water,  snow,  living  animals,  flame,  smoke, 
and  steam.  Among  the  principal  non-conductors  are  gutta  percha,  shellac, 
amber,  the  resins,  sulphur,  glass,  transparent  gems,  silk,  wool,  hair,  feath- 
ers, dry  paper,  leather,  baked  wood,  air,  and  gases  generally. 

Good  conductors,  when  brought  in  contact  with  excited  bodies,  at  once 
draw  off  their  electricity,  and  transmit  it  to  all  parts  of  their  own  surface, 
however  extended.  Bad  conductors,  on  the  other  hand,  receive  electricity 
slowly,  and  diffuse  it  over  their  own  surfaces  no  less  slowly.  A  good  con- 
ductor connected  with  the  earth  or  a  body  of  water,  does  not  for  an  instant 
retain  electricity  communicated  to  it,  but  merely  serves  as  a  highway  for  its 
passage  to  either  of  those  media. 

770.  Insulators. — The  best  non-conductors  are  called 


one  kind  of  electricity  always  accompanied  ?  How  may  this  be  shown  ?  763.  What 
are  Electrics?  Non-electrics?  To  which  of  these  classes  do  the  metals  belong? 
769.  How  may  it  be  shown  that  there  is  a  difference  in  the  conducting  power  of  dif- 
ferent substances?  "What  is  a  Conductor  of  electricity?  A  Non-conductor?  T« 
which  of  these  two  classes  do  the  electrics  generally  belong  ?  To  which,  the  non- 
•lectrics  ?  Mention  some  of  the  chief  conductors.  Some  of  the  principal  non-con- 
ductors. Show  the  difference  between  good  conductors  and  bad  conductors,  when 
brought  in  contact  with  excited  bodies.  What  is  said  of  good  conductors  connected 


294  ELECTRICITY. 

Insulators,  because  they  insulate  electrified  bodies, — that  is, 
cut  off  their  communication  with  such  objects  as  would 
withdraw  their  electricity.  The  air  is  an  insulator ;  were 
it  not,  no  substance  could  remain  electrified  for  an  instant. 
When  insulated,  an  excited  body  retains  the  electricity 
communicated  to  it,  and  is  said  to  be  charged.  The  pith 
bah1  in  the  experiment  described  in  §  758  was  insulated  by 
the  silk  thread.  Had  it  been  suspended  by  a  wire,  the 
metal,  being  a  good  conductor,  would  have  withdrawn 
the  electricity  from  the  ball  as  fast  as  it  was  received,  and 
none  of  the  phenomena  that  followed  would  have  been 
exhibited. 

Even  when  insulated,  excited  bodies  will  in  time  part  with  their  electric- 
ity. This  is  because  no  insulation  can  be  perfect. — Air,  when  imbued  with 
moisture,  acquires  conducting  power ;  and  hence  in  damp  weather  it  is  im- 
possible to  keep  an  electric  excited  for  any  length  of  time.  "Well  insulated 
bodies,  slightly  excited,  may  be  kept  several  months  in  a  dry  atmosphere 
without  any  perceptible  loss  of  electricity.  .  . 

771.  PATH  OF  AN  ELECTRIC  CURRENT. — An  electric  cur- 
rent always  follows  the  best  conductor,  and  of  two  equally 
good  it  takes  the  shorter. 

772.  VELOCITY  OF  ELECTRICITY. — Various  experiments 
have  been  made  to  determine  the  velocity  of  electricity. 
Their  results  show  that  electricity  travels  from  11,000  to 
288,000  miles  in  a  second,  according  to  its  intensity  and 
the  nature  of  the  conductor  along  which  it  passes.     In  the 
case  of  the  velocity  last  mentioned,  which  far  exceeds  that 
of  light,  and  is  so  great  as  to  be  absolutely  inconceivable, 
the  conductor  was  copper  wire. 

773.  ELECTRICAL  MACHINES. — The  Electrical  Machine 
is  an  apparatus  for  developing  large  quantities  of  electricity 
by  the  friction  of  a  rubber  on  a  glass  surface.     Two  kinds 
of  electrical  machines  are  in  use,  known  as  the  Cylinder 

with  the  earth  or  a  body  of  water?  770.  What  is  meant  by  Insulators  ?  Why  ar« 
they  so  called  ?  Give  an  example  of  an  insulator.  When  is  an  excited  body  said  to 
be  charged  T  Give  an  example.  How  is  it  shown  that  no  insulation  is  perfect  ? 
Show  the  difference  in  conducting  power  between  dry  and  damp  air.  771.  What  path 
Is  always  taken  by  an  electric  current  ?  772.  How  great  is  the  velocity  of  electricity  ? 
773.  What  is  the  Electrical  Machine?  How  many  kinds  of  electrical  machines  are 


ELECTKICAL  MACHINES.  295 

and  the  Plate  Machine, — a  glass  cylinder  being  used  in  the 
former,  and  a  circular  plate  of  glass  in  the  latter. 

774.  Experiments  in  electricity  were  originally  performed  with  a  glass 
tube  rubbed  with  fur  or  flannel.  Otto  Guericke,  the  inventor  of  the  air-pump, 
was  the  first  to  contrive  a  machine  for  developing  the  fluid  more  abundantly. 
It  consisted  of  a  globe  of  sulphur,  turned  with  a  winch,  and  submitted  to  tho 
friction  of  the  hand.  Newton  substituted  a  glass  globe  for  the  sulphur. 
About  the  middle  of  the  eighteenth  century,  two  further  improvements  were' 
made, — the  use  of  a  rubber  instead  of  the  hand,  and.  the  addition  of  a  metal- 
lic conductor. 

775.  The  Cylinder  Machine. — In  the  cylinder  machine, 
represented  in  Fig.  268,  electricity  is  developed  by  the  fric- 
tion of  a  rubber  upon  a  glass  cylinder,  usually  from  8  to  12 
inches  in  diameter,  supported  between  two  uprights  of  well- 
dried  wood,  and  made  to  revolve  by  a  couple  of  wheels,  as 
shown  in  the  Figure,  or  (as  is  now  generally  preferred)  by 
a  simple  winch  attached  to  one  end  of  the  cylinder. 

Fig.  268. 


THE   CYLINDER  ELECTRICAL  MACHINE. 


in  use?  What  constitutes  the  difference  between  them?  774.  With  what  were  ex- 
periments in  electricity  originally  performed  ?  Who  first  contrived  an  electrical  ma- 
chine? Describe  Guericke's  apparatus.  What  improvement  did  Newton  make? 
What  improvements  were  made  about  the  middle  of  the  eighteenth  century? 


296  ELECTKICITY. 

A  is  the  cylinder.  The  rubber,  B,  is  a  leather  cushion  stuffed  with  hors« 
hair,  and  set  on  a  spring  which  makes  it  press  equally  against  the  cylinder: 
in  all  parts  of  its  revolution.  The  intensity  of  its  pressure  is  regulated  by 
a  sliding  base-board,  H,  which  can  be  moved  by  a  screw  towards  or  from 
the  cylinder.  Connected  with  the  back  of  the  rubber  is  the  negative  con- 
ductor, F,  a  hollow  metallic  cylinder,  with  round  ends,  insulated  by  a  glass 
pillar.  On  the  opposite  side  is  a  similar  metallic  cylinder,  C,  insulated  in 
the  same  way,  and  called  the  prime  conductor.  Attached  to  this  is  a  rod 
bearing  a  row  of  metallic  points,  E,  like  the  teeth  of  a  rake,  projecting  to- 
wards the  cylinder  and  reaching  to  within  a  short  distance  of  it.  Several 
-holes  of  different  size  are  made  in  the  upper  surface  of  the  prime  conductor, 
to  admit  of  the  introduction  of  different  pieces  of  apparatus  used  in  experi- 
menting. To  prevent  the  electricity  from  escaping  in  the  air  before  it  reaches 
the  prime  conductor,  a  flap  of  black  silk,  G  (which  is  a  non-conductor),  ex- 
tends from  the  upper  edge  of  the  rubber,  across  the  top  of  the  cylinder,  to 
within  an  inch  of  the  metallic  points. 

776.  Operation. — When  the  machine  is  to  be  used,  its  parts  must  be  per- 
fectly clean  and  dry.  The  rubber  is  rendered  more  efficient  by  spreading  on 
it  a  thin  coat  of  an  amalgam  of  zinc,  tin,  and  mercury,  mixed  with  lard.  The 
screw  must  be  adjusted  so  that  the  rubber  may  press  with  moderate  force  on 
the  glass,  and  the  prime  conductor  so  placed  as  to  bring  the  metallic  points 
about  an  eighth  of  an  inch  from  the  cylinder.  If  positive  electricity  is  re- 
quired, the  negative  conductor  must  be  connected  with  the  earth  by  a  me- 
tallic chain.  This  done,  the  handle  is  turned.  The  electricity  naturally 
present  in  the  rubber  is  thus  decomposed,  and  its  positive  part  follows  the 
revolving  glass.  On  its  reaching  the  metallic  points,  the  neutral  electricity 
naturally  present  in  the  prime  conductor  is  decomposed ;  its  negative  ele- 
ment is  attracted  by  the  positive  fluid  of  the  cylinder,  and  rushes  over  the 
metallic  points  to  unite  with  it,  while  its  positive  portion  is  repelled  to  the 
opposite  surface  of  the  conductor.  The  negative  fluid  received  from  the 
prime  conductor  neutralizes  the  positive  fluid  of  the  cylinder ;  but  on  reach- 
ing the  rubber  (which  has  meanwhile  received  a  supply  from  the  earth 
through  the  conducting  chain)  the  .process  is  repeated.  The  prime  con- 
ductor does  not,  therefore,  receive  any  positive  electricity  from  the  cylinder, 
but  is  rendered  strongly  positive  by  having  its  own  negative  fluid  withdrawn. 

If  negative  electricity  is  wanted,  the  chain  connecting  the  machine  with 
the  earth  must  be  attached  to  the  prime  conductor  instead  of  the  negative 
conductor,  and  the  required  electricity  can  then  be  drawn  from  the  latter. 

Water  being  a  good  conductor,  if  the  air  is  damp  the  electricity  is  dissi- 
pated almost  as  soon  as  it  is  developed.  This  may  be  prevented  by  placing 
under  the  cylinder  a  small  box  containing  a  bar  of  red-hot  iron.  The  radia- 
tion of  heat  from  the  bar  keeps  the  atmosphere  around  the  machine  dry. 

7T5.  Hew  is  electricity  developed  in  the  cylinder  machine  ?  With  the  aid  of  Fig.  2C8, 
point  out  the  different  parts  of  the  cylinder  machine.  How  is  the  electricity  prevent- 
ed from  escaping  before  it  reaches  the  prime  conductor  ?  776.  Describe  the  operation 
of  the  cylinder  machine.  If  negative  electricity  is  wanted,  what  must  be  done  ? 
What  is  the  effect  of  dampness  on  the  working  of  the  machine  ?  How  is  this  diffi- 


ELECTEICAL  MACHINES. 


297 


777.  When  the  machine  is  working,  present  your  knuckle 
to  the  prime  conductor ;  a  spark,  accompanied  by  a  sharp 
cracking  sound,  darts  to  your  hand,  producing  a  pricking 
sensation.   This  is  called  the  Electric  Spark.    Any  conductor 
will  draw  off  a  spark ;  but  let  a  non-conductor,  such  as  a 
piece  of  glass,  be  presented,  and  no  spark  will  be  received. 

778.  The  Plate  Machine. — In  the  Plate  Machine,  a  cir- 
cular plate  of  glass  is  used  instead  of  a  cylinder.  The  great- 
est electrical  effects  have  been  produced  with  these  machines. 
Plates  six  and  seven  feet  in  diameter  have  been  employed, 
with  such  power  that  a  spark  from  their  immense  conduc- 
tors is  nearly  suf-  Fig.  269. 

ficient  to  fell  a 
man  to  the  earth. 
The  most  pow- 
erful machine  in 
the  world,  made 
in  Boston,  for 
the  University  of 
Mississippi,  com- 
bines two  plates, 
each  six  feet  in 
diameter. 

Fig.  269  represents 
the  plate  machine  in 
one  of  its  most  con- 
venient and  efficient 
form5?.  A  A  is  the 
plate,  supported  on  an 
axis  between  two  up- 
rights and  turned  by 
the  handle  D.  The 
plate  is  pressed  by 
two  pair  of  elastic  rub- 
bers, fastened  on  the 
inside  of  the  uprights.  THE  PLATE  ELECTRTCAI,  MACHINE. 

culty  removed  ?  111.  When  a  knuckle  is  presented  to  the  prime  conductor,  what 
follows  ?  If  a  non-conductor  is  presented,  what  takes  place  ?  778.  How  is  electricity 
developed  with  the  Plate  Machine  ?  What  is  said  of  the  power  of  plate  machines? 
How  large  plates  are  sometimes  employed?  Give  an  account  of  the  most  power- 
ful electrical  machine  in  the  world.  With  Fig.  269,  describe  the  plate  machine. 


298  ELECTRICITY. 

E  E  E  is  the  conductor,  which  consists  of  three  long  brass  tubes  joined  at 
right  angles,  with  large  balls  at  intervals.  Opposite  the  centre  of  the  plate, 
two  brass  arms,  B,  C,  provided  with  rows  of  teeth,  extend  on  each  side  from 
the  upright  conductor.  The  plate  being  made  to  revolve  by  means  of  the 
handle  D,  the  same  results  follow  as  in  the  case  of  the  cylinder  machine. 

779.  THE  INSULATING  STOOL.  —  The  Insulating  Stool 
consists  of  a  platform  of  well-bake'd  wood,  supported  on 
glass  legs  covered  with  varnish.  A  person  on  the  stool, 
brought  in  connection  with  the  prime  conductor  of  a  ma- 
chine by  holding  in  his  hand  a  chain  proceeding  from  it, 
may  be  charged  with  positive  electricity.  Sparks  may  be 
drawn  from  his  person,  and  his  hair,  if  fine  and  dry,  will 
stand  on  end.  If  he  holds  in  his  hand  a  silver  spoon  full 
of  alcohol,  another  person  not  on  the  stool  may  set  the 
270  spirits  on  fire  by  simply  pre- 

senting his  finger  to  it,  and 
thus  producing  a  spark.  The 
insulating  stool,  is  used  when 
electricity  is  medically  applied. 
780.  THE  DISCHARGER.  — 
The  Jointed  Discharger,  Fig. 
270,  is  an  instrument  with 
which  an  operator  can  dis- 
charge a  conductor  without 
having  any  of  the  electricity 
pass  through  his  person.  It 
consists  of  a  couple  of  curved 
brass  rods,  terminating  in  balls 

THE  JOINTED  DISCHARGER. 


jointed  and  fixed  in  a  socket,  by  which  they  are  attached  to 
a  glass  handle.  The  glass,  being  a  non-conductor,  cuts 
off  communication  with  the  operator's  hand. 

The  Universal  Discharger,  represented  in  Fig.  271,  is 
an  instrument  for  passing  a  charge  of  electricity  through 
any  substance.  Two  wires,  mounted  on  insulating  pillars, 
are  connected  respectively  with  the  positive  and  the  nega- 

779.  Of  what  does  the  Insulating  Stool  consist?  How  is  it  used?  780.  What  is  the 
Jointed  Discharger  ?  Of  what  does  it  consist  ?  What  is  the  Universal  Discharger  ? 


THE  LEYDEN  JAB. 


299 


TIIK  UNIVERSAL  DISCHARGER. 


Fig.  272. 


tive  conductor  of  a  machine.  r\  Fis- 2T1 

The  substance  to  be  operated 
on  is  placed  on  a  stand  be- 
tween two  balls  at  the  ex- 
tremities of  these  wires,  and 
thus  made  a  part  of  the  elec- 
tric circuit  traversed  by  the 
fluid  when  a  discharge  takes 
place. 

781.  THE  LEYDEN  JAR,  OB  VIAL. — The 
Leyden  \li'-den\  Jar  is  a  glass  vessel  used  for 
accumulating  electricity.  It  is  so  called  from 
having  been  first  used  at  Leyden,  Holland,  in 
the  year  1745. 

The  ordinary  Leyden  jar  (Fig.  272)  consists  of  a  glass 
vessel,  coated  inside  and  outside  with  tin-foil,  to  within 
about  three  inches  of  its  mouth.  It  is  closed  with  a  dry 
Garnished  cork,  through  which  passes  a  wire,  terminating 
above  in  a  brass  knob,  and  below  in  a  chain,  which  touches 
the  inner  coating.  If  the  knob  of  such  a  jar  be  held  within 
half  an  inch  of  the  prime  conductor  when  a  machine  is 
Working,  a  succession  of  sparks  will  pass  to  the  knob.  In 
a  short  time  they  cease,  and  the  jar  is  then  said  to  be 
charged.  The  inside  (being  connected  with  the  knob)  is  charged  with  posi- 
tive, and  the  outside  with  negative  electricity,  which  are  prevented  from 
uniting  by  the  non-conducting  glass  between  them. 

If  a  person  now  grasp  the  outside  of  the  jar  with  one  hand,  and  touch  the 
knob  with  the  other,  he  will  experience  the  peculiar  sensation  called  "  the 
electric  shock",  in  his  arms,  and  if  the  jar  is  large,  through  his  chest.  If,  on 
the  other  hand,  he  apply  one  ball  of  the  jointed  discharger  to  the  outer  coat 
and  the  other  to  the  knob,  the  jar  will  be  discharged  without  his  feeling  any- 
thing, because  his  communication  with  the  jar  is  cut  off  by  the  glass  handle. 
A  body  through  which  a  charge  is  to  be  sent  must  form  part  of  the  circuit 
between  the  inner  and  outer  coating  of  the  jar,  so  that  a  union  of  the  positive 
and  the  negative  fluid  can  not  take  place  without  passing  through  it. — So 
much  electricity  is  sometimes  accumulated  in  a  jar  that  a  discharge  takes 
place  through  the  glass,  making  a  hole  in  it  and  rendering  the  jar  useless. 

Describe  it  and  its  mode  of  operation.  781.  What  is  the  Leyden  Jar  ?  Why  is  it  go 
called  ?  Of  what  does  the  ordinary  Leyden  jar  consist  ?  How  is  the  jar  charged  ? 
With  what  kind  of  electricity  is  the  inside  charged?  The  outside?  How  may  the 
dectric  shock  be  taken  ?  How  may  the  jar  be  discharged  without  the  operator's  tak- 
ing a  shock  ?  What  is  essential  in  order  that  a  charge  may  be  sent  through  a  body  f 


LEYDEN   JAR. 


300  ELECTRICITY. 

Any  number  of  persons  may  take  a  shock  at  once.  Having  joined  hands 
BO  as  to  form  a  circle,  let  the  person  at  one  end  take  hold  of  a  chain  connect- 
ed with  the  outside  of  a  jar,  while  the  one  at  the  other  end  touches  the  knob 
with  a  piece  of  wire.  The  painful  sensation  experienced  when  a  shock  is 
taken,  is  caused  by  the  obstructions  which  those  parts  of  the  body  that  are 
imperfect  conductors  present  to  the  free  passage  of  the  electric  fluid. 

782.  An  interesting  incident  is  related  in  connection  with  the  experiments 
that  led  to  the  invention  of  the  Leyden  jar.  Prof.  Muschenbroeck,  of  Ley- 
den,  observing  that  excited  electrics  soon  lose  their  electricity  in  the  air,  de- 
termined to  see  whether  he  could  not  collect  and  insulate  the  fluid  in  a  vessel 
of  non-conducting  glass,  so  that  it  might  be  kept  locked  up.  as  it  were,  ready 
for  use.  Accordingly,  he  introduced  a  wire  from  a  prime  conductor  into  a 
bottle  filled  with  water.  After  the  machine  had  been  working  some  time,  an 
attendant,  holding  the  bottle  in  one  hand,  attempted  to  withdraw  the  wire 
with  the  other,  when  he  of  course  received  a  shock, — so  unexpected  and  so 
unlike  anything  he  had  ever  felt  before,  that  it  filled  him  with  consternation. 
Muschenbroeck  himself  subsequently  took  a  similar  shock,  which  he  de- 
scribed in  a  letter  to  a  French  philosopher.  He  says  that  he  felt  himself 
struck  in  his  arms,  shoulders,  and  breast,  so  that  he  lost  his  breath,  and  it 
was  two  days  before  he  recovered  from  the  effects  of  the  blow  and  the  fright. 
He  would  not,  he  adds,  take  a  second  shock  for  the  whole  kingdom  of  France. 

783.  THE  ELECTRICAL  BATTERY. — When  a  very  heavy 
charge  is  required,  a  number  of  jars,  coated  in  the  usual 
way,  are  placed  in  a  box  lined  with  tin-foil,  which  forms  a 
Fig.  273.  communication  between  their  out- 

er coatings,  while  their  knobs  and 
consequently  their  inside  coatings, 
are  connected  in  the  manner  rep- 
resented in  Fig.  273.  From  its 
powerful  effects,  such  a  combina- 
tion is  called  an  Electrical  Battery. 
By  bringing  one  of  the  knobs  in 
connection  with  a  prime  conductor 
all  the  jars  may  be  charged  as  readily  as  one,  care  being 
taken  to  connect  the  outer  coatings  with  the  earth.  The 
battery  may  be  discharged  in  the  same  way  as  a  single  jar, 
but  the  operator  must  not  let  the  charge  pass  through  his 

What  is  the  consequence  if  too  much  electricity  is  accumulated  in  a  jar  ?  How  may 
any  number  of  persons  take  a  shock  at  once  ?  By  what  is  the  painful  sensation  of  an 
electric  shock  caused?  782.  Eelate  an  incident  connected  with  the  invention  of  the 
Leyden  jar.  What  did  Muschenbroeck  say  of  the  electric  shock  ?  783.  Describe  the 
Electrical  Battery,  and  its  mode  of  operation.  What  effects  may  be  produced  by  the 


THE  ELECTRICAL  BATTEKY. 


ELECTRICAL   EXPERIMENTS. 


301 


Fig.  274. 


person.  The  shock  of  a  powerful  battery  will  kill  a  man 
and  fell  an  ox;  even  moderate  discharges  prove  fatal  to 
birds  and  the  smaller  animals. 

784.  EXPERIMENTS  WITH  THE  ELECTRICAL  MACHINE. — 
With  the  electrical  machine  and  different  pieces  of  appara- 
tus that  accompany  it,  a  variety  of  experiments  may  be 
performed. 

785.  Electrical  Bells.— This  apparatus  (Fig.  274)  il- 
lustrates electrical  attraction  and  repulsion.     Two  bells 
are  suspended  from  a  frame,  with  a  brass  clapper  be- 
tween them.     One  of  these  bells  having  been  placed  in 
connection  with  the  prime  conductor  and  the  other  with 
the  ground,  the  machine  is  worked ;  when  the  former 
becomes  charged  with  positive  and  the  latter  with  neg- 
ative electricity.     The  clapper  is  attracted  to  the  posi- 
tive bell,  strikes  it,  becomes  itself  charged  by  the  con- 
tact, and  is  repelled  till  it  strikes  the  negative  bell.    Its 
positive  electricity  is  there  drawn  off,  and  it  falls  back, 
to  be  again  attracted  and  repelled.     The  clapper  is  thus 
made  to  strike  the  bells  alternately. 

786.  The  Electrical  See-saw.— The  Electrical  See-saw 

(Fig.  275)  operates  on  the  same  principle.  A  brass  beam,  with  a  light  figure 
on  each  end,  is  suspended  on  an  insulating  pillar,  in  such  a  way  as  to  allow 
its  extremities  to  move  freely  up  and 
down.  Two  brass  balls  are  sup- 
ported at  opposite  sides  of  the  stand, 
not  far  from  the  ends  of  the  beam, — 
the  one  on  a  glass  pillar,  the  other 
on  a  metallic  rod.  The  insulated 
ball  is  connected  with  the  inner 
coating  of  a  Leyden  jar,  and  the 
other  with  its  outer  coating.  No 
sooner  is  the  jar  charged  than  the 
figure  near  the  insulated  ball  is  suc- 
cessively attracted  and  repelled,  and 
this  causes  the  beam  to  teeter.  In 
the  same  way  motion  may  be  com- 


ELECTRICAL   BELLS. 


Fig.  275. 


o  o 

ELECTRICAL  BEE-SAW. 

municated  to  a  figure  swinging,  a  floating  swan,  an  insect  suspended  in  the 
air,  &c. 

787.  Dancing  Images. — On  a  metallic  plate  supported  by  some  conducting 


•hock  of  a  powerful  battery  ?  7S5.  Give  an  account  of  the  experiment  with  the  Elec- 
trical Bells.  786.  Describe  the  Electrical  See-saw.  To  what  may  motion  be  com- 
municated on  the  same  principle  ?  787.  Give  an  account  of  the  experiment  with  the 


302 


ELECTRICITY. 


Fig.  277. 


DANCING  IMAGES. 


DIVEBGING   THREADS. 


Fig.  278. 


Fig.  276.  substance,  place  several  light  figures  of  pith  or 

paper,  and  three  or  four  inches  above  them  sus- 
pend another  plate  from  the  prime  conductor. 
As  soon  as  the  machine  is  worked,  the  figures 
will  rise  and  dance  up  and  down  from  one  plate 
to  another  in  a  ludicrous  manner,  as  shown  in 
Fig.  276.  If  the  lower  plate  is  insulated,  when 
they  return  to  it  after  having  been  drawn  up,  th§ 
surplus  positive  electrici- 
ty can  not  escape,  and 
the  dance  ceases. 

788.  Diverging  Threads. 
— Figure  277  represents 
twenty  fine  linen  threads, 
eight  or  ten  inches  long, 
tied  together  at  each  end. 
Attach  them  to  a  prime 
conductor,  and  on  work- 
ing the  machine,  being  all 
filled  with  electricity  of 

the  same  kind,  they  will  repel  each  other  and  assume 

an  oval  form. 

789.  The  Electrified  Head.— On  the  same  prin- 
ciple a  head  of  hair  is  made  to  stand  grotesque- 
ly on  end,  as  shown  in  Fig.  278,  by  fixing  the 
wire  to  which  it  is  attached  in  one  of  the  holes 
of  a  prime  conductor.     The  hairs  are  charged 
with  electricity  of  the  same  kind,  and  are  there- 
fore in  a  state  of  mutual  repulsion.    Fig.  279. 
Draw  off  the  fluid  by  presenting  a 
knife-blade,  and  they  at  once  fall. 

790.  The  Electrical  Pail.— Suspend 
from  the  prime  conductor  by  a  chain 
a  pail  with  a  small  hole  in  the  bot- 
tom, and  fill  it  with  water.    Before 
the  machine  is  worked,  the  water  falls 
from  the  hole  drop  by  drop ;  but,  as 
soon  as  the  water  is  charged  with  elec- 

THE  ELECTRIFIED  HEAD.  tricity,  it  flows  out  in  a  stream,  which 
in  the  dark  seems  to  be  of  fire.  This  is  owing  to  the  repulsion 
excited  in  the  particles  of  water  by  charging  them  with  the  same 
electricity.  ELECTEIO' 

791.  The  Aurora  Tube. — This  apparatus  shows  the  phenomena       PAIL. 

Dancing  Images.  "Why  do  the  images  cease  to  move  if  the  lower  plate  is  insulated  ? 
788.  What  does  Fig.  277  represent  ?  What  takes  place  when  these  threads  are  at- 
tached to  a  prime  conductor  ?  789.  Describe  the  experiment  with  the  llead  of  Hair. 


ELECTRICAL  EXPERIMENTS. 


303 


produced  when  electricity  passes  through  a  vacuum.  It  is  Fig.  280. 
a  glass  tube,  from  two  to  three  feet  long,  surmounted  by  a 
brass  ball.  This  ball  is  supported  on  a  wire,  which  passes 
into  the  tube  through  its  air-tight  top,  and  terminates  a 
short  distance  below  in  a  point.  Inside  of  the  tube,  near 
the  bottom,  is  another  brass  ball  supported  on  a  wire.  The 
lower  part  of  the  tube  is  arranged  so  that  it  can  be  fitted 
to  the  plate  of  an  air-pump,  and  is  commanded  by  a  stop- 
cock. Having  thoroughly  dried  and  warmed  the  tube,  ex- 
haust it  by  means  of  an  air-pump  ;  then,  in  a  dark  room, 
bring  the  upper  ball  in  communication  with  a  prime  con- 
ductor. As  soon  as  the  machine  is  worked,  the  whole 
length  of  the  tube  is  filled  with  a  continuous  stream  of 
violet  light ;  which,  on  a  gmall  scale,  strikingly  resembles 
the  Aurora  Borealis,  or  Northern  Lights.  This  is  a  lumi- 
nous appearance  often  visible  in  the  north  on  clear  and 
frosty  nights,  and  peculiarly  vivid  in  high  latitudes.  It  is 
supposed  that  the  Northern  Lights  are  produced  by  the 
passage  of  currents  of  electricity  through  strata  of  highly 
rarefied  air. 

792.  Luminous  Words. — When  the  con- 
tinuity of  a  conductor  is  broken,  a  spark  darts 
from  one  part  of  it  to  another.  Taking  ad- 
vantage of  this  fact,  we  may  perform  a  vari- 
ety of  experiments,  which  in  a  dark  room  have 
a  striking  effect. 

Fig.  281.  On  a   piece 

of  glass  paste 
some  strips  of 
tin-foil,  with 
portions  cut 

OUt        SO         that       AUKOEA  TUBE. 

the  spaces  may  form  letters,  as 
shown  in  Fig.  281.  Connect  the 
first  piece  of  foil  with  the  prime 
conductor,  and  the  last  with  the 
ground.  When  the  machine  is 
worked,  sparks  will  pass  between 
the  different  divisions  of  the  foil,  and  the  letters  consequently  appear  like 

How  may  the  hairs  be  made  to  fall  ?  790.  Describe  the  experiment  with  the  Electri- 
cal Pail.  "What  causes  the  water  to  flow  more  rapidly  when  the  machine  is  worked  ? 
791.  What  is  shown  with  the  Aurora  Tube  ?  Of  what  does  it  consist  ?  Describe  the 
experiment  with  it.  By  what  is  it  supposed  that  the  Northern  Lights  are  produced  ? 
T92.  What  takes  place  when  the  continuity  of  a  conductor  is  broken?  By  taking  ad- 


304  ELECTRICITY. 

characters  of  fire. — Serpentine  and  spiral  lines  of  light,  and  other  beautiful 
appearances  may  be  produced,  by  arranging  spangles  on  glass  in  the  de- 
sired form  about  one-tenth  of  an  inch  apart,  and  subjecting  them  to  the  ac- 
tion of  the  machine. 

793.  The  Electrical  Pistol. — The  electric  spark  may  be 
made  to  explode  a  mixture  of  hydrogen  and  common  air. 
In  this  experiment  the  Electrical  Pistol  (Fig.  282)  is  em- 
ployed. 

The  barrel  of  the  pistol  is  of  brass. 
Where  the  trigger  is  usually  found,  is  a 
short  ivory  tube,  which  insulates  a  wire 
passing  nearly  across  the  barrel,  and  ter- 
ras ELECTRICAL  PISTOL.  minating  on  the  outside  in  a  ball.  Hold 
the  mouth  of  the  pistol  over  a  stream  of  hydrogen*  gas,  and  when  enough  has 
entered,  close  it  with  a  cork.  On  passing  a  spark  through  the  barrel  from 
the  extremity  of  the  wire  to  the  opposite  surface,  a  loud  report  will  be  pro- 
duced, and  the  cork  will  be  discharged  with  considerable  force. 

794.  MECHANICAL  EFFECTS  OF  THE  PASSAGE  OF  ELEC- 
TRICITY.— A  pointed  conductor  receives  and  parts  with  the 
electric  fluid  much  more  readily  than  one  with  a  spherical 
surface.     Hence,  in  electrical  machines,  points  connected 
with  the  prime  conductor  are  brought  near  the   excited 
glass,  while  the  prime  conductor  itself  is  cylindrical. 

Fix  a  pointed  rod  on  the  prime  conductor,  and  a  silent 
discharge  will  take  place  from  it  as  long  as  the  machine 
is  worked.  In  this  case,  the  prime  conductor  can  not 
accumulate  enough  electricity  to  give  a  spark.  In  a  dark 
room,  the  fluid  is  seen  issuing  from  the  point  in  the  form 
of  a  luminous  brush.  The  electric  current  may  be  felt  if 
the  hand  is  brought  near  the  rod,  and  is  sometimes  strong 
enough  to  blow  out  a  candle.  No  such  phenomena  occur 
near  the  surface  of  the  conductor  or  a  ball  attached  to  it. 
The  point  parts  with  its  electricity  more  readily,  charges 
the  air  in  contact  with  it,  and  repels  it  when  charged,  as  in 
the  case  of  the  pith  ball, — thus  causing  a  constant  current 
from  the  point. 

vantage  of  this  fact,  what  beautiful  experiments  may  be  performed  ?  793.  For  what 
is  the  Electrical  Pistol  used  ?  Describe  this  instrument,  and  the  experiment  per- 
formed with  it.  794  Why,  in  electrical  machines,  are  metallic  points  connected  with 
the  prime  conductor  brought  near  the  excited  glass  ?  Why  ia  the  prime  conductor 
\tself  cylindrical  ?  With  what  experiments  is  the  silent  discharge  from  points  illus- 


THE  PHOSPIIOr.DS   CCP. 


MECHANICAL  EFFECTS   OF 

795.  The  Phosphorm  Cup.— An 
interesting  experiment,  showing 
the  passage  of  an  electric  current, 
may  be  performed  with  the  appara- 
tus represented  in  Fig.  283,  known 
as  the  Phosphorus  Cup.  Two  brass 
cups  insulated  on  glass  pillars  are 
placed  at  the  same  height,  about 
two  inches  apart, with  a  lighted  can- 
dle midway  between  them.  The 
cups,  being  each  provided  with  a 
piece  of  phosphorus,  are  connected 
one  with  the  prime  conductor,  and 
the  other  with  the  negative  conductor,  of  a  powerful  machine.  When  the  ma- 
chine is  worked,  the  flame  sets  in  the  direction  of  the  negative  cup,  towards 
which  it  is  carried  by  the  current  of  positive  fluid  from  the  opposite  cup.  The 
phosphorus  in  the  negative  cup  is  soon  set  on  fire  by  the  heat  thus  pro- 
duced, whereas  at  the  positive  cup  there  is  no  increase  of  temperature,  and 
the  phosphorus  in  it  remains  nnignited.  By  reversing  the  connections  with 
the  machine,  the  opposite  results  may  be  produced,  the  flame  being  always 
carried  towards  the  cup  connected  with  the  negative  conductor. 

796.  When  the  electric  fluid  passes  off  from  a  pointed 
conductor,  the  reaction  may  be  made  to  turn  a  wheel,  and 
thus  set  delicate  machinery  in  motion.  To  exhibit  the  ef- 
fects of  this  reaction,  different  pieces  of  apparatus  have  been 
constructed,  among  Fig.  254. 

which   is  the  Electri- 
cal Flyer. 

The  Electrical  Flyer.— 
The  Electrical  Flyer  consists 
of  a  number  of  brass  wires 
branching  out  from  a  com- 
mon centre,  having  their  ends 
bent  at  right  angles  in  the 
same  direction.  Poise  the 
flyer  on  a  wire  inserted  in  the 
prime  conductor,  and  work 
the  machine.  A  stream  of 
fluid  issues  from  each  point,  and  the  flyer  is  made  to  revolve  in  the  oppo- 
site direction  by  the  reaction  of  the  air.  When  the  room  is  darkened,  the 

trated  ?  Explain  how  a  lighted  candle  is  blown  out  by  an  electric  current.  795.  What 
does  Fig.  283  represent?  Describe  this  apparatus,  and  the  experiment  performed 
with  it.  Towards  which  cup  is  the  flamo  always  carried?  796.  How  may  delicate 
machinery  be  set  In  motion  ?  How  is  this  reaction  shown  ?  Describe  the  Electrical 


TUB   ELECTRICAL  FLYEK. 


306 


ELECTRICITY. 


points  become  luminous,  and  a  circle  of  fire  seems  to  be  formed  as  they 
revolve.        ^~. 

On  the  isame  principle,  horsemen  (mounted  on  the  ends  of  the  flyer)  may 
be  made  to  move  in  a  circle ;  wheels  may  be  turned,  the  sails  of  a  windmill 
set  in  motion,  and  a  light  body  made  to  roll  up  an  inclined  plane. 

797.  The  Thunder  House. — The  power  of  electricity,  as 
a  mechanical  agent,  may  be  further  illustrated  with  an  in- 
genious apparatus  known  as  the  Thunder  House. 


Fig.  285. 


THE  THUNDER  HOUSE. 


The  Thunder  House  consists  of  a  piece  of  baked 
mahogany,  B  B,  shaped  like  the  gable  of  a  house,  and 
attached  to  a  stand.  Down  the  centre  runs  a  wire,  C, 
terminating  above  in  a  ball,  A.  Several  square  pieces, 
D,  F,  about  one-fourth  of  an  inch  thick,  are  cut  out  of 
the  gable,  and  placed  loosely  in  the  holes  from  which 
they  are  cut.  Across  each  square  passes  a  wire  in  such 
a  direction  that  by  inserting  the  squares  one  way  we 
have  an  uninterrupted  line  from  C  to  E  ;  but  putting 
them  in  crosswise,  we  break  the  continuity  of  the  con- 
ductor at  D  and  P.  Connect  the  end  of  the  wire,  E, 
with  the  outside  of  a  Leyden  jar  j  and,  having  inserted 
the  square  so  that  the  conducting  line  may  be  un- 
broken, pass  a  charge  through  the  wire  by  connecting 
the  ball  A  with  the  inside  of  the  jar.  A  report  will  be 
heard,  but  neither  of  the  loose  pieces  will  be  displaced. 
Now  let  one  of  the  pieces  remain  in  the  same  position,  and  place  the  other 
crosswise ;  then,  on  passing  a  powerful  charge  through  the  wire,  the  former 
will  remain  undisturbed,  while  the  latter  will  be  thrown  out  of  the  gable  by 
the  mechanical  action  of  the  fluid  in  leaping  over  the  break. 

798.  Among  the  mechanical  effects  of  an  electric  dis- 
charge may  be  mentioned  the  perforation   of  thin  non- 
conducting substances,  such  as  a  card  or  a  piece  of  paper. 
Glass  one-twelfth  of  an  inch  thick  may  be  pierced  by  a  dis- 
charge from  a  powerful  battery. 

799.  THE  ELECTRIC  SPAEK. — The  color  of  the  electric 
spark  varies  according  to  the  medium  through  which  it 
passes.      In  ordinary  air  and  oxygen,  it  is  bluish  white ; 
in  rarefied  air,  violet ,   in  nitrogen,  a  purplish  blue ;    in 
hydrogen,  crimson ;  in  carbonic  acid  and  chlorine,  green. 

Flyer.  What  is  the  effect  of  darkening  the  room  ?  To  what  may  motion  be  com- 
municated on  the  principle  of  the  flyer  ?  797.  What  apparatus  further  illustrates  the 
mechanical  power  of  electricity  ?  Describe  the  Thunder  House,  and  the  experiment 
performed  with  it,  798.  What  other  mechanical  effect  of  an  electric  discharge 
!a  mentioned  ?  799.  What  does  the  color  of  the  electric  spark  depend  on  ?  What 


/ 


^ 

d  onyfthe 


THE  ELECTKIC  SPAEK* 
y 

The  length  and  intensity  of  the  spark  < 
electrical  intensity  of  the  body  from  whic&  nT^ 
Sparks  may  be  taken  from  the  prime  conductor  oY^5 
erful  machine  at  a  distance  of  more  than  two  feet.  * 
given  machine,  the  positive  conductor  yields  much-  ] 
powerful  sparks  than  the  negative. 

800.  Ignition  by  the  Electric  Spark. — Inflammable 
stances  may  be  set  on  fire  by  the  electric  spark,  as  is  shown 
by  several  experiments. 

Stand  on  the  insulating  stool,  touch  the  prime  conductor  with  one  hand, 
and  from  the  other  transmit  a  spark  to  a  burner  from  which  a  current  of  gas 
is  issuing,— the  gas  will  be  ignited.  In  houses  thoroughly  dried  by  furnace 
heat,  persons,  by  simply  running  over  the  carpet,  have  been  sufficiently 
charged  with  electricity  to  light  gas  with  a  spark  from  the  finger. — Present  a 
candle  just  extinguished,  with  its  wick  still  glowing,  to  a  prime  conductor, 
so  that  a  spark  may  pass  through  the  snuff  to  the  candle,  and  it  will  be  re- 
lighted.— A  person  on  an  insulating  stool  charged  with  electricity  may  set 
fire  to  a  cup  of  ether  by  presenting  to  it  an  icicle,  through  which  the  spark 
is  transmitted.— With  a  suitable  apparatus,  a  fine  wire  may  be  melted  by 
sending  through  it  a  charge  from  a  powerful  battery. 

801.  The  Electrical  Fire  House. 
— Rosin  may  be  ignited  with  the 
apparatus  known  as  the  Electrical 
Fire  House  (Fig.  286).  Brass  wires, 
insulated    by  being    enclosed   in 
glass  tubes,enter  the  opposite  sides 
of  the  house,  and  terminate  on  the 
inside  in  two  knobs,  B,  C,  a  short 
distance  apart.    These  knobs  are 
loosely    covered   with   tow    and 
sprinkled   with  powdered    rosin. 
When  a  charge  is  passed  from  A 
to  D,  the  rosin  is  ignited,  and  the 
flame  seen  through  the  windows 
gives  the  house  the  appearance  of 
being  on  fire. 

802.  Apparatus  for  firing  Crunpowder. — This  apparatus  consists  of  two 

is  its  color  in  ordinary  air  and  oxygen ?  In  rarefied  air?  In  nitrogen?  In  hydro- 
gen ?  In  carbonic  acid  and  chlorine  ?  What  do  the  length  and  intensity  of  the  spark 
depend  on?  At  what  distance  have  sparks  been  taken  from  a  powerful  machine? 
How  do  the  sparks  from  the  positive  conductor  compare  with  those  from  the  nega- 
tive? 800.  What  is  the  effect  of  the  electric  spark  on  inflammable  substances? 
Prove  this  with  several  experiments.  What  is  the  effect  of  sending  a  powerful 
•harge  through  a  fine  wire  ?  801.  Describe  th-j  Electrical  Fire  House,  and  the  ex- 


Fig.  2S6. 


THE  ELECTRICAL  FIRE  HOUSE. 


X 


>^ 
c 


308  ELECTRICITY. 

Fig.  287.  insulating  glass  pillars  fixed  in  a  stand, 

to  one  of  which  is  attached  a  wire  termi- 
nating in  a  ball,  to  the  other  a  wooden 
cup  for  holding  the  powder.  The  chains 
c,  d,  being  connected  respectively  with 
the  inner  and  outer  surface  of  a  Leyden 
jar,  a  spark  is  made  to  pass  from  b  to  A, 
which  ignites  the  powder. 

803.  THE  ELECTKOPHORUS. 
— Small  quantities  of  electricity 
maybe  accumulated  with  a  sim- 
ple apparatus  known  as  the  Electrophorus,  which  to  a  cer- 
tain extent  answers  as  a  substitute  for  the  electrical 
machine. 

The  electrophorus  consists  of  a  cake  of  a  resinous  mixture  8  or  10  inches 
in  diameter,  and  a  somewhat  smaller  plate  of  metal  with  a  rounded  edge  and 
a  glass  handle,  by  which  it  may  be  raised  without  drawing  off  the  electricity. 
Excite  the  resinous  mixture  with  fur,  and  placing  on  it  the  metallic  plate, 
touch  the  upper  surface  of  the  latter  for  an  instant  to  let  its  negative  elec- 
tricity escape.  Then  raise  the  metallic  plate  by  the  insulated  handle,  and 
on  presenting  a  conductor  a  spark  will  be  given.  Place  the  metallic  plate 
again  upon  the  rosin,  and  on  raising  it  another  spark  may  be  withdrawn.  A 
Leyden  jar  may  thus  be  slowly  charged.  Left  on  the  rosin,  the  metallic 
plate  will  remain  charged  for  a  long  time,  and  may  be  conveniently  used  as 
occasion  requires  in  experimenting. 

804.  ELECTROSCOPES. — Electroscopes  are  instruments 
for  detecting  the  presence  of  electricity,  and  determining 
whether  it  is  positive  or  negative.  They  appear  in  various 
forms, — the  simplest  being  the  pith  ball  suspended  by  a 
silk  thread,  represented  in  Fig.  266.  The  attraction  of  the 
pith  ball  in  its  natural  state  by  any  substance  presented  to 
it,  indicates  the  presence  of  electricity  in  the  latter.  When 
the  pith  ball  is  charged  with  positive  electricity,  its  attrac- 
tion by  any  substance  indicates  negative  electricity  in  the 
latter,  and  its  repulsion  positive.  When  the  pith  ball  is 

periment  performed  with  it.  802.  Of  what  does  the  apparatus  for  firing  gunpowder 
consist  ?  803.  With  what  may  small  quantities  of  electricity  be  accumulated  ?  Of 
what  does  the  Electrophorus  consist ?  How  is  it  worked?  804.  What  are  Electro- 
scopes ?  What  is  the  simplest  form  of  the  electroscope  ?  How  is  the  presence  of  elec- 
tricity in  any  substance  indicated?  When  the  pith  ball  is  positively  charged,  what 
Joes  its  attraction  by  any  substance  indicate  ?  What,  its  repulsion  ?  When  the  pith 


THE  ELECTROMETER. 


309 


Fig.  283. 


QTTADKANT  ELEC- 
TIIOMETEH. 


charged  with  negative  electricity,  its  attraction  by  any  sub- 
stance indicates  positive  electricity  in  the  latter,  its  repul- 
sion negative. 

805.  ELECTROMETERS. — Electrometers  are 
instruments  for  measuring  approximately  the 
quantity  of  electricity  in  a  given  conductor 
or  other  body.     Electrometers,  more  or  less 
sensitive,  are  made  in  different  forms ;  one  of 
the  simplest  is  the  Quadrant  Electrometer, 
shown  in  Fig.  288. 

A  slender  ivory  rod,  with  a  pith  ball  attached  to  its 
lower  end,  is  suspended  from  a  wooden  pillar  so  as  to 
swing  freely  like  a  pendulum.  The  pivot  on  which  it 
turns  is  the  centre  of  a  semicircular  scale  attached  to  the 
pillar ;  and  the  whole  apparatus  terminates  in  a  brass  pin 
which  may  be  inserted  in  the  top  of  a  prime  conductor. 
The  greater  the  quantity  of  electricity  in  the  latter,  the 
farther  from  the  pillar  the  pith  ball  will  swing, — and  this 
distance  is  indicated  by  the  scale. 

806.  ELECTRICAL  INDUCTION. — An  electrical  atmosphere 
surrounds  every  excited  body.      An  insulated  conductor 
situated  within  this  atmosphere  becomes  excited,  and  when 
thus  affected  is  said  to  be  electri-  Fiw  m 

fied  by  induction.  The  phenom- 
ena of  electrical  induction  are  con- 
stantly exhibited. 

807.  Electrical  induction  is  illustrated  with 
the  apparatus  represented  in  Fig.  289.  c  a  d  is 
a  brass  cylinder  with  rounded  ends,  insulated 
on  a  glass  support  and  furnished  at  one  ex- 
tremity with  a  pith  ball  electroscope,/.  On 
bringing  the  end  d  within  a  few  inches  of  a 
prime  conductor,  the  pith  balls,  which  be- 
fore hung  close  together,  instantly  separate, 
indicating  the  presence  of  electricity.  Since 
the  cylinder  is  not  in  contact  with  the  prime  INDUCTION  APPARATUS. 

ball  is  negatively  charged,  what  does  its  attraction  indicate  ?  What,  its  repulsion  ? 
805.  What  are  Electrometers  ?  What  is  one  of  the  simplest  forms  called  ?  Describe 
the  Quadrant  Electrometer,  and  its  mode  of  operation.  806.  By  what  is  every  ex- 
eited  body  surrounded  ?  When  is  a  body  said  to  be  electrified  "by  induction  f 
807.  Describe  the  apparatus  for  illustrating  electrical  induction,  and  the  experiments 


310  ELECTRICITY. 

conductor  and  receives  no  sparks  from  it,  it  is  obviously  electrified  by  induc- 
tion. Its  neutral  and  latent  electricity  is  decomposed  by  the  electrical  at- 
mosphere which  surrounds  the  prime  conductor:  the  negative  portion  is 
attracted  towards  d,  and  the  positive  repelled  to  c,  where  it  charges  the  two 
balls,  and  thus  causes  them  to  separate.  If  the  cylinder  is  removed  from 
the  neighborhood  of  the  prime  conductor,  the  pith  balls  immediately  fall  to- 
gether ;  it  is  only  when  within  the  atmosphere  of  the  prime  conductor  that 
they  indicate  any  electrical  excitement. 

If  the  cylinder  cad,  instead  of  being  insulated,  is  connected  with  the 
earth,  its  positive  electricity  is  driven  off  to  the  latter,  while  the  negative 
portion  is  retained.  If  the  cylinder  is  then  removed,  its  communication  with 
the  earth  being  first  cut  off,  it  will  remain  excited  with  negative  electricity. 

808.  ELECTKICITY  FROM  STEAM. — Electricity  is  devel- 
oped during  the  escape  of  steam  from  an  orifice.     This  fact 
was  discovered  in  1840  by  a  workman  attending  a  steam- 
engine  ;  who,  happening  to  take  hold  of  the  safety-valve 
with  one  hand  while  the  other  was  in  a  jet  of  steam  escap- 
ing from  a  fissure,  received  an  electric  shock.     The  experi- 
ment was  repeated,  and  it  was  found  that  a  person  with 
one  hand  in  a  jet  of  escaping  steam  could  give  a  shock  with 
the  other  to  any  one  in  contact  with  the  boiler  or  the  brick 
work  supporting  it.   The  electricity  in  question  is  produced 
by  the  friction  of  minute  particles  of  water  against  the  sides 
of  the  orifice. 

As  soon  as  this  fact  came  to  the  knowledge  of  scientific  men,  an  appara- 
tus known  as  the  Hydro-electric  Machine  was  invented  for  the  purpose  of 
experiment.  It  consists  of  a  steam  boiler  from  three  to  six  feet  long,  mount- 
ed on  insulating  pillars,  with  an  arrangement  for  letting  the  steam  escape  in 
jets  against  a  plate  covered  with  metallic  points,  which  acts  like  a  prime  con- 
ductor. This  machine  develops  electricity  in  prodigious  quantities,  its  power 
being  equal  to  that  of  four  large  plate  machines  combined.  It  yields  sparks 
twenty-two  inches  long,  in  such  quick  succession  that  they  resemble  a  sheet 
of  flame. 

809.  ATMOSPHERIC  ELECTRICITY. — The  atmosphere,  be- 
sides the  neutral  and  latent  electricity  which  resides  in  it 
as  in  all  other  substances,  contains  more  or  less  free  elec- 


performed  with  it?  How  may  the  cylinder  be  charged  with  negative  electricity? 
808.  Under  what  circumstances  is  electricity  produced  by  steam  ?  State  the  circum- 
stances attending  this  discovery.  What  was  found  when  the  experiment  was  repeat- 
ed? Howiathe  electricity  in  question  produced?  What  instrument  was  invented 
Cor  the  sake  of  further  experiment  ?  Describe  the  Hydro-electric  machine.  To  wnat 
ts  its  power  equal?  What  is  said  of  its  sparks?  809.  What  does  the  atmosphere 


ATMOSPHERIC  ELECTBICITY.  311 

tricity,  the  quantity  increasing  with  the  distance  from  the 
earth's  surface.  This  is  proved  by  sending  up  arrows  con- 
nected by  a  conducting  metallic  wire  with  a  delicate  elec- 
trometer.  The  higher  the  arrows  rise,  the  more  the  elec- 
trometer is  affected.  An  experimenter  in  England,  by 
connecting  a  number  of  pointed  conductors  with  an  insu- 
lated wire  a  mile  long  and  raised  a  hundred  feet  above  the 
earth's  surface,  has  collected  enough  electricity  to  charge 
a  battery  of  fifty  jars  every  three  seconds. 

810.  Origin. — The  free  electricity  in  the  atmosphere  is 
due — 1.  To  the  friction  of  large  masses  of  air  of  different 
densities  on  each  other.     2.  To  the  condensation  of  atmos- 
pheric vapors  into  a  liquid  form — a  process  which  develops 
electricity  in  great  abundance.   3.  To  the  chemical  changes 
involved  in  the  growth  of  trees  and  plants.     4.  To  evapo- 
ration^ particularly  in  the  case  of  water  filled  with  vegetable 
matter  undergoing  decomposition. 

As  these  processes  are  not  always  going  on  with  the 
same  activity,  it  follows  that  the  quantity  of  free  electricity 
present  in  the  atmosphere  differs  at  different  times  and 
places. 

811.  St.  Elmo^s  Fire. — When  the  atmosphere  is  very 
abundantly  charged  with  electricity,  its  presence  is  indi- 
cated by  various  luminous  phenomena.    Hence  the  brilliant 
light  called  St.  Elmo's  Fire,  which  frequently  appears  at 
night  on  the  tops  of  masts,  the  points  of  bayonets,  and  the 
tips  of  the  ears  of  horses.     It  is  simply  the  superabundant 
electricity  of  the  atmosphere,  attracted  by  a  pointed  con- 
ductor, into  which  it  silently  passes.     Such  phenomena  are 
most  common  during  thunder-storms,  when  as  many  as 
thirty  have  been  seen  in  different  parts  of  the  same  vessel. 
Sometimes  they  resemble  sheets  of  flame,  and  extend  three 
feet   in  length ;   at   others  they  take  the  form  of  globes 


contain?  To  what  is  the  free  electricity  in  the  atmosphere  proportioned?  How  is 
this  proved  ?  What  has  been  done  in  this  connection  in  England  ?  810.  To  what 
four  processes  is  the  free  electricity  in  the  atmosphere  chiefly  due  ?  Why  is  the  quan- 
tity of  free  electricity  in  the  atmosphere  different  at  different  times  ?  811.  When  are 
luminous  phenomena  observed  in  the  atmosphere?  Describe  the  phenomenon  known 


312  ELECTEICITY. 

of  fire,  attaching  themselves  to  the  yard-arms  and  mast- 
heads. 

812.  Fire-balls. — To  electricity  are  also  attributable  the 
Fire-balls  which  are  from  time  to  time  observed  darting 
through  the  atmosphere,  at  heights  of  thirty  miles  and  up- 
wards, and  with  velocities  of  from  five  to  thirty-three  miles 
in  a  second.   These  balls  sometimes  vanish  suddenly,  leaving' 
behind  them  a  luminous  track ;  at  other  times  they  explode 
into  smaller  balls  or  sparks  ;  and  at  others  again  they  are 
accompanied  with  showers  of  meteoric  stones.     Falling  or 
shooting  stars  are  the  same  phenomena  on  a  smaller  scale, 
and  in  lower  regions  of  the  atmosphere. 

813.  Lightning  and  Thunder. — The  grandest  of  all  the 
phenomena  produced  in  the  atmosphere  by  electricity  is 
Lightning.      Lightning  is  nothing  more  than  the   spark 
which  accompanies  the  passage  of  the  electric  fluid  from 
one  cloud  to  another,  or  between  a  cloud  and  the  earth. 
Thunder  is  the  crackling  sound  produced  at  the  same  time 
by  the  sudden  rush  of  air  into  the  vacuum  which  the  elec- 
tric fluid,  as  it  darts  with  inconceivable  rapidity,  leaves 
behind  it.     Flashes    of  lightning  are   sometimes   several 
miles  in  extent ;  and,  as  the  crackling  sound  is  produced 
at  every  point  of  their  course,  it  does  not  reach  our  ear  all 
at  the  same  instant.     Hence  the  rolling  or  rumbling  of 
thunder,  which  is  in  some  cases  prolonged  by  successive 
echoes  from  neighboring  mountains  or  clouds. 

814.  That  lightning  and  thunder  are  produced  by  an 
electric  discharge,  though  previously  suspected,  was  first 
experimentally  proved  in   1752,   by  Benjamin  Franklin, 
whom  the  world  recognizes  alike  great  as  a  philosopher 
and  a  patriot. 

Impressed  with  the  conviction  that  lightning  and  the  electric  spark  were 
identical,  Franklin  determined  to  test  its  truth  by  trying  to  collect  electricity 

as  St.  Elmo's  Fire.  At  what  time  is  it  most  common  ?  What  different  forms  does  it 
assume  ?  812.  What  other  phenomena  are  attributable  to  electricity  ?  What  be- 
comes of  these  fire-balls  ?  What  are  shooting  stars  f  813.  What  is  the  grandest  of 
all  the  electrical  phenomena  of  the  atmosphere  ?  What  is  Lightning  ?  What  is 
Thunder?  How  is  the  rolling  of  thunder  accounted  for?  814.  By  whom  and  when 
was  it  proved  that  lightning  and  thunder  are  produced  by  an  electric  discharge  ? 


FRANKLIN'S  EXPERIMENT.  313 

from  the  clouds  during  *  thunder-storm.  With  this  view  he  made  arrange- 
ments for  extending  a  wire  to  a  great  height  from  a  steeple  then  in  course  of 
erection  in  Philadelphia.  The  work  advanced  but  slowly ;  and  while  anx- 
iously watching  its  progress  one  day,  he  observed  a  boy's  kite  far  up  in  tho 
air,  and  higher  than  he  could  hope  to  get  his  wire  even  when  the  steeple 
should  be  finished.  It  struck  him  at  once  that  with  this  simple  toy  he  could 
make  the  desired  experiment,  letting  the  string  perform  the  part  of  the  con- 
ducting wire.  Accordingly,  he  made  a  cross  of  two  strips  of  cedar,  to  the 
extremities  of  which  he  fastened  the  four  corners  of  a  silk  handkerchief, 
using  this  as  a  covering  that  his  kite  might  be  able  to  withstand  the  rain  and 
wind  accompanying  a  thunder-shower.  A  sharp-pointed  wire  extended  a 
foot  from  the  top  of  the  cross,  to  draw  off  the  electricity  from  the  clouds. 

The  kite  thus  constructed  was  raised  by  Franklin  and  his  son  in  the  first 
thunder-storm  that  occurred  in  June,  1752.  Hempen  twine  was  used,  at  the 
lower  end  of  which  a  key  was  fastened  for  a  prime  conductor,  while  the  whole 
was  insulated  by  a  silk  ribbon  fastened  to  a  non-conductor  sheltered  from  the 
wet.  With  intense  anxiety  the  philosopher  awaited  the  result.  .A  cloud 
passed  without  any  electrical  indications,  and  he  began  to  despair  of  success. 
Another  came,  and  now  to  his  indescribable  joy  he  saw  the  loose  fibres  of 
the  twine  stand  out  every  way  and  follow  his  finger  as  it  passed  to  and  fro. 
Presenting  his  knuckle  to  the  key,  he  received  a  spark  ;  and  as  soon  as  the 
twine  waa  wet  with  rain,  and  its  conducting  power  thus  increased,  the  elec- 
tricity was  abundant.  A  Leyden  jar  was  charged  from  the  key,  with  which 
spirits  were  set  on  fire,  and  other  experiments  performed. — This  discovery 
raised  its  author  to  the  first  rank  among  the  philosophers  of  his  day.  His 
own  feelings  at  the  triumphant  result  of  his  experiment  may  be  imagined. 
"  Convinced  of  an  immortal  name,  he  felt  he  could  have  been  content  if  that 
moment  had  been  his  last." 

Franklin's  experiment  was  repeated  with  success  in  various  parts  of  Eu- 
rope. There  was  no  room  left  for  doubting  the  identity  of  lightning  with 
the  electric  spark.  In  later  times  this  identity  has  been  further  confirmed  by 
phenomena  connected  with  the  electric  telegraph.  Reports  as  loud  as  that 
of  a  pistol  are  often  heard  in  telegraph  offices  during  a  storm,  and  to  ensure 
the  safety  of  the  operators  the  wires  have  to  be  connected  by  conductors 
with  the  earth.  Even  in  clear  weather  it  is  sometimes  found  difficult  to  fix 
the  wires  on  the  poles,  in  consequence  of  numbness  produced  in  the  hands 
by  electricity  conducted  to  them  by  the  wires. 

815.  Effects  of  Lightning. — Lightning  produces  both 
mechanical  and  chemical  effects.  Its  mechanical  effects  are. 
very  powerful.  It  crushes  huge  trees,  rends  off  their 
branches,  and  sometimes  tears  their  trunks  into  fragments. 

Relate  the  incidents  connected  with  Franklin's  great  discovery.  "What  was  the  re- 
sult of  this  experiment  as  regards  the  reputation  of  its  author  ?  As  regards  his  own 
feelings?  "Where  was  the  experiment  repeated ?  How  has  the  identity  of  lightning 
with  the  electric  spark  been  since  confirmed?  815.  Mention  some  of  the  mechanical 

14 


314  ELECTRICITY. 

When  buildings  are  struck,  large  masses  of  masonry  are 
displaced;  a  brick  wall  more  than  12  feet  long  has  been 
carried  in  one  piece  to  a  distance  of  15  feet.  These  effects 
are  analogous  to  the  throwing  out  of  the  blocks  of  wood 
from  the  gable  of  the  Thunder  House,  as  described  in 
§  797.  It  is  only  (as  shown  in  that  experiment)  in  the  case 
of  imperfect  conductors, — that  is,  when  obstructions  are 
presented  to  the  free  passage  of  the  electric  fluid, — that 
these  effects  are  produced. 

Lightning  is  also  a  powerful  chemical  agent.  It  decom- 
poses water  and  other  substances  into  their  elements.  It 
sets  fire  to  trees  and  houses,  and  melts  metallic  bodies. 
On  the  tops  of  mountains  it  is  not  unusual  to  see  the  sur- 
face of  the  hardest  rocks  perforated  with  deep  cavities 
covered  with  a  vitreous  crust,  owing  to  their  having  been 
struck  with  lightning. 

816.  Lightning  Hods. — When  a  cloud  becomes  heavily 
charged  with  electricity,  if  another  cloud  in  a  different 
electrical  state  is  near  it,  a  discharge  takes  place  between 
the  two ;  in  which  case  there  is  no  danger.  But  some- 
times there  is  no  such  adjacent  cloud,  and  a  flash  of  light- 
ning darts  from  the  charged  cloud  to  the  earth  or  sea :  it 
is  then  said  to  strike.  In  such  a  case,  the  air  being  a  bad 
conductor,  the  electric  fluid  in  its  descent  follows  any  bet- 
ter conductor  it  can  find,  such  as  a  house,  a  tree,  the  mast 
of  a  ship,  a  living  animal,  or  a  human  being.  Now,  if  the 
objects  just  mentioned  were  perfect  conductors,  the  light- 
ning would  follow  them  to  the  earth  without  doing  any 
injury  ;  but  they  all  offer  some  obstruction  to  its  passage, 
and  therefore  all  suffer  more  or  less  when  struck. 

The  tallest  objects,  reaching  nearest  to  the  clouds,  are  the  most  likely  to 
be  struck.  It  is  therefore  imprudent  to  stand  on  the  top  of  a  hill  or  near  a 
tree  during  a  thunder-storm.  In  the  house  it  is  best  at  such  a  time  not  to 
sit  near  a  damp  wall,  a  bell  wire,  a  gilded  picture  frame,  or  any  metallic  sub- 
effects  of  lightning.  Only  in  what  ease  are  these  effects  produced  ?  State  some  of 
the  chemical  effects  of  lightning.  816.  When  does  an  electric  discharge  take  place 
between  two  clouds?  When,  between  a  cloud  awA  the  earth?  Why  are  houses, 
trees,  &c.,  struck  ?  Why  do  they  suffer  damage  when  struck  ?  What  objects  are 
most  likely  to  be  struck  ?  What  positions  is  it  imprudent  to  take  during  a  thunder* 


LIGHTNING  BODS. 


315 


stance,  as  the  electric  fluid  is  sure  to  select  the  best  conductor  in  its  path  to 
the  earth  if  the  house  should  be  struck. 

817.  Having  proved  lightning  to  be  an  electric  dis- 
charge, Franklin  proceeded  to  devise  means  for  preserving 
buildings  from  its  effects.  He  thus  became  the  inventor 
of  the  Lightning  Rod,  a  simple  contrivance  which  has  been 
instrumental  in  saving  life  and  property  to  an  extent  that 
can  not  be  estimated. 

The  best  material  for  a  lightning  rod  is  copper,  but  iron  is  cheaper  and 
generally  preferred.  It  must  extend  at  least  four  feet  above  the  building  to 
be  protected,  and  terminate  above  in  one  or  more  sharp  points,  which  should 
be  tipped  with  silver  or  platinum  to  keep  them  from  rusting,  and  thus 


losing  part  of  their  conducting  power.  The  rod  should  be 
continuous,  and  of  such  size  that  the  fluid  may  follow  it  freely 
without  danger  of  melting  it,— say  three-fourths  of  an  inch 
across.  It  should  be  placed  as  close  as  possible  to  the  wall 
and  fixed  securely  to  it.  The  lower  end  should  be  divided 
into  two  or  more  pointed  branches,  as  shown  at  a,  a,  a, 
in  Fig.  290.  These  branches  should  slant  away  from  the 
building,  and  at  least  one  of  them  should  sink  far  enough  into 
the  ground  to  reach  water  or  soil  that  is  moist.  If  the  build- 


Fig.  290. 


Fitc.  291. 


ing  is  large,  and  particu- 
larly if  it  has  more  than 
one  point  projecting  up- 
ward, it  should  have  sev- 
eral rods,  either  descend- 
ingdirectly  to  the  ground, 
like  c,  d,  in  Fig.  291,  or 
connected  together  by  a 
good  conductor,  and  ul- 
timately carried  down 
like  e,  f,  g,  7i.  * 

818.  The  security  afforded  by  lightning  rods  is  twofold.  In  the  first  place, 
terminating  in  points,  they  generally  draw  off  the  electric  fluid  silently ;  and 
secondly,  if  a  discharge  takes  place,  the  lightning  in  its  descent  will  follow 
them  rather  than  the  inferior  conductors  to  which  they  aro  attached,  and 
finding  a  free  passage  through  them  will  do  no  injury. — Lightning  rods  have 
not  been  found  efficacious  to  a  greater  distance  than  forty  feet.  Within  this 
limit,  they  protect  a  space  around  themselves  equal  to  twice  the  height  that 


Btorm  ?  817.  Who  invented  the  Lightning  Eod  ?  Of  what  materials  is  the  lightning 
rod  made  ?  What  should  be  its  form  and  size,  to  ensure  the  safety  of  a  building  ? 
In  what  case  should  a  building  have  several  rods?  How  may  they  in  that  case  be 
arranged  ?  818.  In  what  two  ways  do  lightning  rods  conduce  to  the  safety  of  a  build- 
ing ?  What  is  the  greatest  distance  at  which  lightning  rods  have  been  found  effica- 


31Q  ELECTRICITY. 

they  project  above  the  building ;  for  example,  a  rod  projecting  five  feet  will 
protect  every  point  of  the  surrounding  surface  within  ten  feet  of  itself. 

819.  ELECTRICAL  FISH. — The  torpedo,  the  Surinam  eel, 
the  si-lu'-rus  electricus,  and  several  other  gpecies  of  fish,  have 
a  peculiar  organ  with  which  they  can  give  electric  shocks, 
more*  or  less  powerful  according  to  their  size.     They  use 
this  organ  for  defending  themselves  against  enemies,  and 
for  stunning  and  thus  securing  their  prey.     The  power  of 
giving  shocks  ceases  with  life ;  its  too  frequent  exerciso 
exhausts  the  fish  and  ultimately  kills  it.     The  shock  of  a 
torpedo  fourteen  inches  long  is  borne  with  difficulty ;  and 
the  Surinam  eel  has  been  found  of  such  size  that  its  shock 
proved  immediately  fatal. 

The  Surinam  eel  gives  as  many  as  twenty  shocks  a  minute,  yields  the 
electric  spark  in  the  air,  and  charges  a  Leyden  jar.  Faraday  computed  that 
the  average  shock  of  one  of  these  eels  on  which  he  experimented  was  equal 
to  the  discharge  of  a  battery  of  fifteen  jars,  containing  3,500  square  inches  of 
glass,  charged  as  heavily  as  possible. — The  South  American  Indians  catch 
these  eels  by  driving  a  number  of  wild  horses  into  a  pond  "containing  them. 
The  eels,  roused  from  their  muddy  retreats,  vigorously  defend  themselves 
by  pressing  against  the  stomachs  of  the  horses  and  repeatedly  discharging 
their  electrical  battery.  The  poor  beasts,  panting  from  their  struggles,  with 
mane  erect  and  haggard  eyes  expressing  fright  and  anguish,  seek  to  escape 
from  their  invisible  foes,  but  are  driven  back  by  the  Indians  who  surround 
the  pond,  armed  with  long  reeds,  and  making  terrible  outcries.  After  sev- 
eral of  the  horses  are  stunned  and  drowned  the  eels  become  exhausted  by 
their  continued  discharges,  and  are  no  longer  objects  of  dread  to  the  Indians. 
Slowly  approaching  the  shore,  they  are  captured  with  harpoons  fastened  to 
long  cords ;  and  to  such  a  degree  is  their  electrical  power  weakened  that 
hardly  any  shock  at  all  is  received  in  drawing  them  ashore. 

The  silurus  is  a  fish  twenty  feet  long,  found  in  the  Nile  and  the  Niger ; 
its  electrical  apparatus  lies  immediately  below  the  skin  and  extends  round 
the  whole  body. 

Voltaic  Electricity; 

OR,  ELECTRICITY  PRODUCED   BY   CHEMICAL   ACTION. 

820.  Having  considered  electricity  produced  by  fric- 
tion, we  proceed  to  treat  of  that  developed  by  chemical 

cious?  Within  this  limit,  how  great  a  space  do  they  protect?  819.  What  species  of 
fish  have  the  power  of  giving  an  electric  shock  ?  For  what  purposes  do  they  use  this 
power  ?  What  is  the  effect  of  its  too  frequent  exercise  ?  What  is  said  of  the  shock 
of  a  torpedo  fourteen  inches  long  ?  Of  the  Surinam  eel  ?  What  was  the  power  of  ono 
experimented  on  by  Faraday  ?  How  do  the  South  American  Indians  capture  these 


VOLTAIC  ELECTRICITY.  317 

action.  This  branch  of  the  subject  is  known  as  Gal- 
vanism. 

821.  GALVANI'S  DISCOVERY  AND  THEORY. — The  first 
discoverer  in  this  department  of  science  was  he  from  whom 
it  received  its  name,  Galvani  [gal-vah'-ne].  Professor  of 
Anatomy  in  the  University  of  Bologna,  Italy.  The  effects 
of  atmospheric  electricity  on  the  animal  frame  had  long 
engaged  his  attention.  In  the  year  1790,  having  prepared 
the  hind  legs  of  some  frogs  suitably  for  experiment,  and 
hung  them  on  copper  hooks  till  they  should  be  needed,  he 
observed  to  his  surprise,  on  accidentally  pressing  the  lower 
extremities  against  the  iron  railing  of  a  balcony,  that  they 
were  drawn  up  with  a  singular  convulsive  action.  He 
found  upon  experiment  that  similar  contortions  were  pro- 
duced whenever  copper  and  iron,  connected  with  each 
other,  were  brought  in  contact,  the  one  with  the  nerves  of 
the  thigh,  the  other  with  the  muscles  of  the  leg. 

Galvani's  experiment  is  often  repeated  at  the  present  day.  To  perform 
it,  separate  the  lower  extremities  of  a  frog  from  the  rest  of  the  body,  skin 
them,  and  pushing  back  the  muscles  on  either  side  of  the  back-bone,  lay  bare 
the  lumbar  nerves.  Stretching  out  the  _,v  2_2 

legs  in  the  position  shown  in  Fig.  292, 
lay  a  thin  curved  rod  of  zinc  under  the 
nerves,  and  touch  the  muscles  of  the 
leg  with  a  similar  rod  of  copper.  As 
long  as  the  rods  are  kept  apart,  there  is 
no  movement  in  the  legs ;  but  the  in- 
stant they  are  brought  in  contact,  a  vi- 
olent convulsive  motion  takes  place,  the 
legs  are  drawn  into  the  position  shown 
by  the  dotted  lines,  and  these  contor- 
tions are  repeated  as  often  as  the  rods 
are  separated  and  again  brought  to- 
gether. 

Galvani  attributed  this  convulsive 
movement  to  a  certain  vital  fluid  which  he  supposed  to  reside  in  the  nerves, 
and  to  pass  to  the  muscles  over  the  metallic  conductors,  in  a  manner  similar 
to  the  passage  of  electricity  between  the  inner  and  the  outer  coating  of  a 

eels  ?  What  is  said  of  the  silurus  ?  820.  What  Is  Galvanism  ?  821.  From  whom  did 
it  receive  its  name  ?  Give  an  account  of  Galvani's  discovery.  How  may  Galvani'a 
experiment  be  repeated  at  the  present  day  ?  When  do  the  contortions  take  place  ? 
To  what  did  Galvani  attribute  this  convulsive  movement  ?  What  did  he  call  thin 


3  38  VOLTAIC  ELECTRICITY. 

Leyden  jar  when  it  is  discharged.  He  therefore  called  this  supposed  fluid 
Animal  Electricity ;  but  in  compliment  to  its  discoverer  it  soon  became  known 
as  Galvanic  Electricity,  or  the  Galvanic  Fluid. 

822.  VOLTA'S  THEORY  AND  THE  VOLTAIC  PILE. — Prof. 
Volta,  of  Pavia,  experimenting  further  on  the  subject,  soon 
laid  aside  Galvani's  theory  of  a  "  vital  fluid  ",  and  held  that 
the  effects  in  question  were  caused  by  the  contact  of  the 
two  dissimilar  metals ;  that  the  legs  of  the  frog  had  no 
agency  in  producing  the  galvanic  excitement,  but  merely 
gave  indications  of  its  presence,  like  the  pith  ball  electro- 
scope in  the  case  of  ordinary  electricity.     To  prove  this,  he 
combined  the  metals  apart  from  all  animal  organizations  ; 
and  advancing  step  by  step,  about  the  year  1800,  he  gave 
to  the  world  his  celebrated  PILE,  the  appearance  of  which 
marked  a  new  era  in  the  history  of  electrical  science. 

Volta's  "contact  theory"  was  at  one  time  generally  received;  but  it  is 
now  known  that  the  galvanic  excitement  is  not  produced  by  the  mere  con- 
tact of  the  metals,  but  by  chemical  action.  A  third  element,  such  as  the 
moisture  of  the  hand,  animal  fluids,  an  acid,  or  some  saline  solution,  must 
act  chemically  on  one  of  the  metals.  It  is  believed  that  no  chemical  action 
ever  takes  place  without  the  development  of  free  electricity,  though  the  quan- 
tity may  be  so  small  as  to  escape  our  senses. 

823.  Yolta's  Pile  consisted  of  a  number  of  circular  plates 
of  copper  and  zinc,  and  pieces  of  cloth  moistened  with  a 
weak  acid  or  saline  solution,  alternating  as  follows,  the  same 
order  being  observed  throughout.     At  the  base  of  the  pile 
was  a  plate  of  copper,  and  on  this  a  zinc  plate,  the  two 
constituting  a  pair.   On  this  pair  was  a  piece  of  cloth  moist- 
ened as  above,  then  a  second  similar  pair  (the  copper  al- 
ways below),  then  a  piece  of  cloth,  a  third  pair,  and  so  on 
to  the  top  of  the  pile.     The  whole  was  insulated  on  glass, 
and  a  wire  was  attached  to  each  end.    The  wire  connected 
with  the  zinc  plate  at  the  top  of  the  pile  yielded  positive 
electricity;  that  connected  with  the  copper  plate  at  the 
base,  negative.   When  the  ends  of  these  wires  were  brought 

supposed  vital  fluid  ?  What  other  names  were  soon  given  to  it  ?  822.  "Who  experi- 
mented further  on  the  subject  ?  State  Volta's  theory.  To  what  invention  did  Volta's 
investigations  lead  ?  What  is  now  thought  of  Volta's  "  contact  theory  "  ?  With  what 
b  chemical  action  always  accompanied  ?  823.  Of  what  did  Volta's  Pile  consist  ?  De- 


VOLTA'S  PILE.  319 

together  or  separated,  a  bright  spark  was  produced.  A 
very  fine  platinum  wire,  half  an  inch  long,  stretched  between 
the  ends  of  the  wires,  was  made  red  hot.  A  person  taking 
one  of  these  wires  in  each  hand,  received  a  succession  of 
shocks,  like  those  from  a  Ley  den  jar,  but  slighter, — their 
intensity  depending  on  the  number  of  plates.  These  effects 
were  produced  as  long  as  the  arrangement  and  condition 
of  the  plates  remained  unchanged. 

Volta's  pile,  immediately  connected  as  it  was  with  the  Galvanic  Battery 
(which  has  since  superseded  it),  was  one  of  those  inventions  to  which  science 
is  most  largely  indebted.  It  has  immortalized  its  author,  in  honor  of  whom 
this  species  of  excitement  produced  by  chemical  action  is  now  generally 
called  Voltaic  Electricity. 

824.  FAMILIAR  EXPERIMENTS. — The   effects  of  voltaic 
electricity  may  be  illustrated  with  familiar  experiments. 

Experiment  1. — Place  a  piece  of  zinc  under  the  tongue,  and  on  the  tongue 
a  silver  coin.  As  long  as  the  metals  do  not  touch,  nothing  is  perceived ; 
but  as  soon  as  they  are  brought  in  contact,  the  voltaic  circuit  is  formed,  a 
thrilling  sensation  is  felt  in  the  tongue,  a  taste  somewhat  like  copperas  is 
perceived,  and,  if  the  eyes  are  closed,  a  faint  flash  of  light  is  seen.  Here 
electricity  is  developed  by  the  chemical  action  of  the  saliva  upon  the  zinc. 

Exp.  2. — Lay  a  silver  dollar  on  a  sheet  of  zinc,  and  on  the  coin  place  a 
living  snail  or  leech.  No  sooner  does  the  creature  in  moving  about  get 
partly  off  the  dollar  and  on  the  zinc,  than  it  receives  a  shock  and  re» 
coils.  In  this  case  it  is  the  slime  of  the  snail  or  leech  that  acts  chemically  on 
the  zinc. 

825.  GALVANIC  BATTERIES. — Soon  after  inventing  the 
pile,  Volta  proposed  another  arrangement  for  the  metallic 
plates,  identical  in  principle,  but  more  convenient  for  use. 
He  discovered  that   electrical   excitement  was   exhibited 
whenever  slips  of  copper  and  zinc  were  immersed  in  a  ves- 
sel containing  some  diluted  acid,  if  the  circuit  was  com- 
pleted by  bringing  the  metals  themselves,  or  wires  con- 
nected with  them,  in  contact  above  the  vessel.     Such  an 
arrangement  is   called   a   Simple   Galvanic   Circle  ;    it  is 


«cribe  some  of  its  effects.  How  long  were  these  effects  produced  ?  What  is  said  of 
the  invention  of  Volta's  Pile  ?  What  is  electricity  produced  by  chemical  action  now 
generally  called  ?  824.  What  is  the  first  experiment  with  which  the  effects  of  voltaio 
electricity  are  familiarly  illustrated  ?  The  second  experiment  ?  825.  Soon  after  in- 
Tenting  the  pile,  what  discovery  did  Volta  make  ?  What  is  such  an  arrangement 


320 


VOLTAIC   ELECTRICITY. 


SIMPLE  GALVANIC 

CIRCLE. 


Fig.  293.  shown  in  Fig.  293.     Combining  a  num- 

ber of  vessels  similarly  prepared,  Volta 
made  the  first  galvanic  battery,  known 
as  the  Couronne  des  Tasses  \koo-rone'  da 
tahs\. 

826.  The  Couronne  des  Tasses,  or  "  crown  of  cups  ", 
represented  Fig.  294. 

in  Fig.  294, 
consisted  of 
any  number 
of  vessels, 
each  con- 
tainingaslip 
of  copper 
and  zinc,  the 
copperofone 

vessel  being  COTTEONNE  DES  TASSES. 

connected  by  a  conductor  with  the  zinc  of  the  next. 
To  complete  the  circuit,  wires  attached  to  the  extreme 
metallic  slips  of  the    series  were  brought  together, 
when  a  spark  and  other  electrical  phenomena  were  produced. 

827.  Trough  Battery. — Instead  of  the  separate  cups  used  by  Volta,  one 
long  vessel  divided  into  cells  was  subsequently  employed.     The  zinc  and 
copper  plates,  connected  in  pairs  by  a  slip  of  metal,  and  arranged  at  such 
Fig.  295.  distances  as  to  enclose  a  partition  between  the  zinc 

and  copper  of  each  pair,  were  fastened  to  a  common 
frame,  so  that  they  could  all  be  immersed  in  acid  and 
thus  subjected  to  chemical  action  at  the  same  time. 
This  improved  arrangement  was  known  as  the  Trough 
Battery. 

828.  Smee's  JRattery.—Smee's  Battery  (see  Fig.  295) 
has  three  metallic  plates  suspended,  without  touching 
each  other,  from  a  wooden  frame.  The  middle  plate  is 
of  silver  coated  with  platinum.  The  outside  ones  are 
of  amalgamated  zinc, — that  is,  zinc  coated  with  mer- 
cury. The  whole  are  immersed  in  dilute  sulphuric 
acid  contained  in  an  earthenware  vessel.  No  action 
takes  place  till  communication  is  established  between 
the  metals,  when  a  bubbling  immediately  commences 
in  the  liquid,  and  voltaic  electricity  is  produced.  This 
BMEK  s  BATTERY  battery,  though  not  so  powerful  as  those  hereafter  de- 


called?  What  name  was  given  to  the  first  galvanic  battery,  made  by  Yolta? 
S2G.  Describe  ths  Couronne  des  Taxses.  827.  Describe  the  Trough  Battery.  82&  Do- 
scribo  Smee's  Battery.  What  are  the  advantages  of  this  battery  ?  For  what  is  it 


DANIELL'S  CONSTANT  BATTERY.  321 

scribed,  is  economical,  may  be  kept  in  operation  for  several  days,  and  is  much 
used  in  plating  the  inferior  metals  with  gold  and  silver.  With  certain  mod- 
ifications it  is  also  employed  in  working  the  magnetic  telegraph. 

829.  In  the  batteries  thus  far  described  but  one  fluid 
was  used,  and  two  metals  of  such  a  nature  that  one  was 
more  readily  acted  on  by  the  fluid  than  the  other.  Dilute 
sulphuric  acid  being  used  as  the  fluid,  zinc  (which  it  readily 
acts  upon)  was  generally  taken  for  one  of  the  metals. 
Great  improvements  have  been  made  on  these  single  fluid 
batteries.  With  the  exception  of  Smee's,  they  have  been 
entirely  superseded  by  instruments  in  which  two  fluids  are 
employed,  and  which  are  not  only  more  powerful,  but  also 
more  regular  and  permanent  in  their  action.  The  most 
important  of  these  we  proceed  to  describe. 

830.  Daniell's  Constant  Battery. — The  two-fluid  batteries  are  Fig.  296. 
all  modifications  of  Daniell's,  which  was  invented  in  1836.  It  con- 
sists of  an  outer  cylinder  of  copper,  within  which  is  a  cup  of  un- 
glazed  porcelain,  of  the  shape  represented  in  Fig.  296.  Within 
this  cup  is  a  solid  cylinder  of  amalgamated  zinc.  From  both  the 
zinc  and  the  copper  cylinder  project  brass  cups  (see  Fig.  297)  pro- 
vided with  screws  for  the  insertion  of  wires  ;  the  extremities  of 
which,  if  there  be  but  one  cell,  are  called  the  Poles  of  the  bat- 
tery. If  there  be  several  cells,  strips  of  metals  inserted  in  these 
cups  connect  the  zinc  of  one  with  the  copper  of  the  next,  and 
wires  for  conducting  the  fluid  are  attached  to  the  zinc  of  one  of 
the  extreme  cells  and  the  copper  of  the  other.  The  porous  cup 
is  filled  with  dilute  sulphuric  acid.  The  copper  cylinder  is  filled 
with  the  same  fluid  saturated  with  sulphate  of  copper ;  and  on  a 
perforated  shelf  near  its  top  (represented  by  the  circular  dotted 
lines  in  the  figure)  is  placed  some  of  the  solid  sulphate,  that  as. 
fast  as  this  substance  is  used  up  by  the  chemical  action  a  fresh 
supply  may  be  obtained,  and  the  operation  of  the  battery  thus  x — i^-^ 
made  constant. 

As  soon  as  the  poles  are  joined,  a  powerful  action  commences,  which, 
instead  of  constantly  diminishing  as  in  the  single  fluid  batteries,  is  main- 
tained for  hours  without  losing  any  of  its  efficiency.  For  ordinary  use 
two  dozen  such  cells  are  combined  in  a  battery.  One  of  the  chief  im- 
provements in  this  apparatus  is  the  introduction  of  the  porous  cup,  which 

used  ?  829.  In  tho  batteries  thus  far  described,  what  are  employed  for  the  purpose 
of  producing  chemical  action  ?  Which  is  the  most  efficient  of  the  single  fluid  batte-' 
ries  ?  How  do  the  single  fluid  batteries  compare  with  those  in  which  two  fluids  are 
used  ?  830.  By  whom  and  when  was  the  first  two-fluid  battery  invented  ?  Describe 
Daniell's  Constant  Battery,  and  its  mode  of  operation.  What  is  one  of  the  chief  im- 

14* 


322 


VOLTAIC  ELECTRICITY. 


keeps  the  liquids  apart,  yet  does  not  prevent  the  passage  of  voltaic  cur- 
rents. 

831.  Groves  Battery. — Grove's  Battery  is  the  most  powerful  one  yet  con- 
structed. It  operates  on  the  same  principle  as  Daniell's,  but  employs  differ- 
ent metals  and  fluids,  which  render  it  more  active.  The  porous  cup  contains 
a  strip  of  platinum  immersed  in  strong  nitric  acid,  and  is  itself  contained  in 


Fig.  298. 


a  zinc  cylinder  filled  with  dilute 
sulphuric  acid.  The  whole  is 
set  in  a  vessel  of  glass  or  earth- 
enware. Fig.  298  shows  one  of 
Grove's  batteries  consisting  of 
six  cells,  as  arranged  by  Benja- 
min Pike,  jr.,  of  New  York. 
The  platinum  of  each  cup  is  con- 
nected with  the  zinc  of  the  next. 
At  the  extremities  of  the  cir- 
cuit, wires  are  attached  respec- 
GROVE'S  BATTEBT.  lively  to  the  platinum  of  one 

cell  and  the  zinc  of  the  other,  the  former  of  which  exhibits  positive  electricity 
and  the  latter  negative. 

Grove's  battery  is  the  best  for  performing  the  more  striking  experiments 
of  galvanism,  being  nearly  twenty  times  as  powerful  as  a  zinc  and  copper 
battery  containing  the  same  amount  of  metallic  surface.  Its  superiority  is 
owing  to  the  absorption  of  the  hydrogen  evolved,  the  high  conducting  power 
of  the  fluids  employed,  and  the  ease  with  which  nitric  acid  is  decomposed. 

832.  Buvseria  £attery.—Thei  cost  of  platinum  renders  Grove's  apparatus 
expensive.  Bunsen  therefore  devised  a  battery,  in  which  plates  of  carbon 
acted  on  by  nitric  acid  are  substituted  for  platinum.  In  other  respects  it  is 
like  Grove's,  but  it  is  less  efficient. 

833.  DRY  PILES. — Feeble  galvanic  currents  may  be  pro- 
duced by  compressing  a  great  number  of  circular  pieces  of 
copper  and  zinc  paper  (sometimes  called  gold  and  silver 
paper),  placed  back  to  back,  in  a  varnished  glass  tube, 
which  they  exactly  fit.  As  in  Volta's  pile,  the  same  order 
must  be  observed  throughout.  The  electrical  excitement 
produced  by  a  Dry  Pile  (as  such  an  apparatus  is  called) 
lasts  a  long  time.  Bells  have  been  kept  constantly  ringing 
for  eight  years  by  the  alternate  attraction  and  repulsion  of 
a  clapper  suspended  between  two  such  piles. 


provements  in  this  apparatus  ?  831.  Describe  Grove's  Battery.  How  does  it  com- 
pare in  power  with  a  zinc  and  copper  battery  ?  To  what  is  its  superiority  owing  ? 
832.  What  is  the  objection  to  Grove's  battery  ?  To  remove  this,  what  modification 
did  Bunsen  propose  ?  833.  How  are  Dry  Piles  formed  ?  What  evidence  is  adduced 


THEORY   OP  THE   GALVANIC  BATTERY.  323 

834.  QUANTITY  AND  INTENSITY. — The  quantity  of  vol- 
taic electricity  produced  by  a  battery,  depends  on  the  size 
of  the  metallic  plates   employed ;   its  intensity,  on  their 
number. 

The  difference  between  the  quantity  and  the  intensity  of  the  electric  fluid 
is  analogous  to  the  difference  between  the  quantity  of  a  solid  dissolved  in  a 
given  liquid  and  the  strength  of  the  solution.  Into  a  hogshead  of  water 
throw  a  wine-glass  full  of  salt,  and  into  a  tea-spoon  full  of  water  put  as  much 
gait  as  it  will  dissolve.  The  former  solution  will  contain  a  greater  quantity 
of  salt  than  the  latter,  but  it  will  be  less  strong. 

835.  THEORY  OP  THE  GALVANIC  BATTERY. — Let  us  now 
inquire  how  electricity  is  developed  with  the  galvanic  bat- 
tery.    Take,  as  an  example,  Volta's  single  fluid  apparatus. 
When  the  zinc  and  copper  plates  are  immersed  in  acidu- 
lated water,  and  connection  is  established  between  them, 
the  water  is  decomposed  into  its  elements,  oxygen  and  hy- 
drogen.    The  oxygen  combines  with  the  zinc,  for  which  it 
has  a  strong  affinity,  and  forms  oxide  of  zinc ;  while  the 
hydrogen  appears  about  the  copper  in  the  form  of  minute 
bubbles.     The  zinc,  in  consequence  of  the  chemical  change 
produced  in  its  surface,  parts  with  its  positive  electricity  to 
the  liquid,  and  remains  negatively  electrified.    The  copper, 
not  acted  on  by  the  liquid  as  the  zinc  is,  attracts  from  it 
this  same  electricity,   and  becomes  positively  electrified. 
The  acid  mixed  with  the  water  tends  to  dissolve  the  oxide 
of  zinc  as  fast  as  it  is  formed,  and  thus  to  keep  a  fresh  sur- 
face of  the  metal  exposed  to  the  liquid. 

836.  The  terminal  wires  of  a  battery,  or,  when  no  wires 
are  attached,  the  plates  from  which  they  would  proceed, 
are  called  its  Poles.     The  pole  connected  with  the  metal 
most  easily  acted  on  by  the  fluid,  always  exhibits  negative 
electricity;  the  other,  positive.     For  pole  some  substitute 
the  term  electrode,  meaning  the  path  by  which  a  voltaic 
current  enters  or  leaves  a  body.     The  positive  pole  they 

of  the  permanency  of  their  action  ?  834  On  what  does  the  quantity  of  voltaic  elec- 
tricity produced  by  a  battery  depend  ?  On  what,  its  intensity  ?  Illustrate  the  differ- 
ence between  the  quantity  and  the  intensity  of  the  electric  fluid.  835.  Give  the  the- 
ory of  the  operation  of  the  galvanic  battery.  836.  What  is  meant  by  the  Poles  of  a 
battery  ?  Which  pole  exhibits  negative  electricity  ?  Which,  positive  ?  What  term 


324  VOLTAIC  ELECTRICITY. 

call  the  Anode  (ascending  or  entering  path)  ;  the  negative, 
the  Cathode  (descending  or  departing  path).  When  the 
electrodes  are  brought  in  contact,  the  galvanic  circuit  is 
said  to  be  closed.  The  two  currents  then  meet  and  neu- 
tralize each  other ;  but,  as  fresh  currents  are  all  the  time 
being  produced,  the  action  continues  without  interruption. 

837.  DIFFERENCE  BETWEEN  FKICTIONAL  AND  VOLTAIC 
ELECTKICITY. — Voltaic   electricity  and  that   developed  by 
friction  are  the  same  hi  kind,  but  are  characterized  by  cer- 
tain points  of  difference. 

1.  The  electricity  developed  by  friction  is  far  more  in- 
tense ;  that  produced  by  chemical  action  is  far  greater  in 
quantity. 

A  simple  galvanic  circle  (§  825)  develops  as  much  electricity  iii  three  sec- 
onds as  would  be  accumulated  in  a  battery  of  Leyden  jars  by  thirty  turns  of 
a  powerful  plate  machine.  Yet  so  weak  is  this  voltaic  electricity  that  a  per- 
son receiving  it  through  his  system  would  hardly  be  aware  of  its  passage, 
while  the  same  quantity  from  the  Leyden  jars  might  prove  fatal  t»  life.  It 
takes  a  galvanic  battery  of  about  fifty  pair  of  plates  (no  matter  what  their 
size)  to  affect  a  delicate  electroscope,  and  one  of  nearly  a  thousand  pair  to 
make  pith  balls  diverge. 

2.  The  voltaic  fluid  will  not  pass  through  an  insulating 
medium,  as  the  electric  spark  does.   If  the  circuit  is  broken, 
all  action  at  once  ceases.     It  will  pass  thousands  of  miles 
over  a  conducting  wire,  but  will  not  leap  a  break  the  fiftieth 
part  of  an  inch. 

3.  The  chemical  effects  of  the  voltaic  fluid  are  incom- 
parably greater  than  those  of  frictional  electricity. 

The  galvanic  battery  produces  the  most  intense  heat,  and  readily  decom- 
poses compound  substances ;  no  such  effects  belong  to  the  electrical  machine. 
An  ordinary  galvanic  battery  will  decompose  a  grain  of  water  into  oxygen 
and  hydrogen.  To  do  this  with  frictional  electricity  would  require  the  power 
of  an  electrical  plate  having  a  surface  of  32  acres, — which  would  be  equiva- 
lent to  a  flash  of  lightning. 

838.  EFFECTS  OF  VOLTAIC  ELECTRICITY. — Among  the 

is  by  some  substituted  for  pole?  What  is  the  Anode  ?  What  is  the  Cathode  ?  When 
is  the  galvanic  circuit  said  to  be  closed  t  What  then  takes  place  ?  887.  What  is  the 
first  point  of  difference  between  frictional  and  voltaic  electricity  ?  State  some  facts 
illustrating  this  difference.  What  is  the  second  point  of  difference  between  frictional 
*nd  voltaic  electricity  ?  The  third  point  of  difference  ?  What  facts  are  stated  in  the 


DECOMPOSING  EFFECTS.  325 

effects  of  voltaic  electricity  on  substances  "brought  within 
the  circuit,  may  be  mentioned  the  following  : — 

839.  Decomposition. — Compound  substances  may  be 
decomposed  into  their  elements  with  the  galvanic  battery ; 
and  it  is  a  singular  fact,  that  of  the  elements  so  obtained 
some  always  arrange  themselves  about  the  positive  pole,  and 
others  about  the  negative.  Thus,  oxygen,  chlorine,  iodine, 
and  the  acids,  invariably  fly  to  the  positive  pole,  when  set 
free  from  any  compound  substance  ;  hydrogen,  the  oxides, 
and  the  alkalies,  to  the  negative.  As  the  elements  must  be 
in  an  opposite  electrical  state  to  the  poles  that  attract  them, 
we  conclude  that  oxygen,  chlorine,  &c.,  are  naturally  neg- 
ative,— and  hydrogen,  the  oxides,  and  alkalies,  positive. 
Every  chemical  compound  seems  to  consist  of  a  positive 
and  a  negative  element,  held  together  by  electrical  at- 
traction. 

The  great  discovery  that  water  could  be  decomposed  by  voltaic  electricity 
was  made  in  1800,  immediately  after  the  announcement  of  Volta's  pile,  by 
an  experimenter,  who  observed  that  gas  bubbles  rose  when  the  terminal 
wires  were  immersed  in  water.  Several  years  later,  Davy,  after  a  long  course 
of  experiments,  decomposed  the  earths  and  alkalies,  which  had  before  been 
universally  regarded  as  simple  substances,  and  thus  brought  to  light  a  num- 
ber of  new  metals,  the  existence  of  which  had  not  even  been  suspected. 

840.  The  decomposition  of  water  is  effected  with  Fig.  299. 

the  apparatus  represented  in  Fig.  299.  A  large  glass 
goblet  has  a  frame  fitted  to  its  rim,  from  which  are 
suspended  two  small  receivers  for  the  purpose  of  col- 
lecting the  two  gases  evolved.  As  water  consists  of 
two  parts  of  hydrogen  to  one  of  oxygen,  one  of  the 
receivers  should  be  twice  as  large  as  the  other.  Two 
holes  in  the  bottom  of  the  vessel,  to  which  screw 
cups  are  attached,  admit  the  electrodes  from  a  bat- 
tery, and  terminate  on  the  inside  in  strips  of  plat- 
inum, which  enter  the  receiver.  The  vessel  being 
filled  with  water  and  the  battery  set  in  operation,  de- 
composition at  once  commences.  Oxygen  passes  to  the  positive  electrode 

text  to  illustrate  this  difference  ?  839.  What  is  the  first  effect  of  voltaic  electricity  ? 
What  singular  fact  is  stated  respecting  the  elements  thus  obtained  ?  What  elements 
go  to  the  positive  pole  ?  What,  to  the  negative  ?  What  is  inferred  from  this  fact  ? 
"When  and  under  what  circumstances  was  it  discovered  that  water  could  be  decom- 
posed by  voltaic  electricity  ?  What  great  discovery  was  made  by  Davy  ?  840.  De- 
•cribe  the  mode  of  decomposing  water  with  the  galvanic  battery.  How  is  the  process 


pin* 
HJ1H" 


326  VOLTAIC   ELECTRICITY. 

(which  should  be  inserted  in  the  smaller  receiver)  and  hydrogen  to  the  nega« 
tive.  The  identity  of  the  gases  may  be  proved  by  subsequently  experiment- 
ing on  them.  As  water  is  not  a  very  good  conductor  of  voltaic  electricity, 
the  process  is  facilitated  by  the  addition  of  a  little  sulphuric  acid. 

Fig.  300.  841.  The  decomposition  of  a  neutral  salt  may  be  performed 

with  the  apparatus  represented  in  Fig.  300.  A  glass  tube 
shaped  like  a  V  is  fitted  at  each  end  with  a  cork  and  screw. 
Through  these  screws  pass  the  wires  from  a  battery,  termi- 
nating inside  in  platinum  strips.  The  tube  having  been  filled, 
with  a  solution  of  sulphate  of  soda  or  any  other  neutral  salt, 
colored  blue  with  tincture  of  violets,  the  battery  is  set  in  ac- 
£~  ^^  tion.  No  sooner  is  a  current  passed  from  pole  to  pole  through 

the  liquid,  than  the  latter  is  decomposed.  The  acid  passes  to  the  positive 
pole,  and  the  alkali  to  the  negative.  This  is  shown  by  the  change  of  color 
produced,  the  liquid  becoming  red  around  the  positive  wire  and  green  around 
the  negative.  If  the  poles  be  transposed,  the  effects  will  be  reversed. 

842.  The  decomposing  power  of  the  galvanic  battery  is 
tamed  to  practical  account  in  the  various  processes  of 
ELECTRO-METALLURGY.  This  is  the  art  of  depositing  on 
any  substance  a  coating  of  metal  from  a  metallic  solution 
decomposed  by  voltaic  electricity.  One  of  the  branches 
of  this  art  is  Plating,  which  consists  in  covering  the  inferior 
metals  with  a  thin  coat  of  gold  or  silver.  When  the  metal 
coating  is  not  to  adhere  permanently  to  the  surface  on 
which  it  is  deposited,  but  to  form  a  copy  of  it  and  be  re- 
moved, the  process  is  called  Electrotyping. 

The  different  processes  of  Electro-metallurgy  differ 
somewhat  in  their  details  and  in  the  apparatus  employed, 
but  the  principle  involved  is  the  same  in  all ;  viz.,  that  any 
compound  metallic  solution  is  decomposed. by  the  passage 
through  it  of  a  voltaic  current ;  whereupon  the  pure  metal 
is  attracted  to  the  negative  pole,  while  the  substance  be- 
fore combined  with  it  goes  to  the  positive.  A  medal,  an 
engraving,  or  any  conducting  substance,  has  therefore  only 
to  be  attached  to  the  negative  pole,  and  the  metal  in  ques- 
tion will  be  deposited  on  it,  the  thickness  of  the  coat  de- 

facilitated  ?  841.  With  what  apparatus,  and  how,  may  a  neutral  salt  be  decomposed? 
842.  How  is  the  decomposing  power  of  the  galvanic  battery  turned  to  practical  ac- 
count? What  is  Electro-metallurgy?  In  what  does  Plating  consist?  In  what, 
Electrotyping  ?  What  is  the  principle  involved  in  all  the  processes  of  electro-metal- 
lurgy ?  When  any  conducting  substance  is  attached  to  the  negative  pole,  wbiit  takes 


ELECTKOTYPING.  327 

pending  on  the  length  of  time  it  is  left  to  the  action  of  the 
battery. 

Reversed  copies  are  thus  obtained ;  the  minutest  indentations  on  the  sur- 
face of  the  original  being  represented  by  elevations  on  the  copy,  and  projec- 
tions on  the  original  by  corresponding  indentations  in  the  copy.  If  an  exact 
and  not  a  reversed  copy  is  wanted,  a  mould,  taken  from  the  original  in  wax 
or  plaster,  must  be  submitted  to  the  above  process. 

This  metallic  deposit  will  take  place  only  on  a  good  conductor ;  if,  there- 
fore, the  object  to  be  copied  is  not  such,  it  must  be  endowed  with  conducting 
power  by  dusting  over  it  some  fine  plumbago.  On  the  contrary,  if  there  is 
any  part  of  which  a  copy  is  not  wanted,  it  may  be  covered  with  varnish 
which  is  a  non-conductor. — That  the  copy  may  be  readily  removed  from  the 
original,  the  surface  of  the  latter  should  be  rubbed  with  oil  or  powdered 
plumbago. 

843.  The  most  convenient  mode  of  electrotyping  is  as  follows : — Fill  a 
trough  with  a  solution  of  sulphate  of  copper,  and  over  its  top  extend  two  par- 
allel rods  of  wood  a  short  distance  apart.    Run  the  positive  wire  from  a  bat- 
tery along  one  of  these  rods,  and  the  negative  along  the  other.     From  the 
negative  wire  suspend  in  the  fluid  the  object  to  be  copied,  and  from  the  posi- 
tive one  a  piece  of  copper  plate.     Sulphate  of  copper  is  composed  of  sulphu- 
ric acid  and  copper.     When  the  battery  begins  to  operate,  this  fluid  is  de- 
composed ;  the  copper  is  drawn  to  the  negative  pole  and  deposited  on  the 
object  attached  to  it.    The  sulphuric  acid  goes  to  the  copper  plate,  and 
combining  with  it  forms  sulphate  of  copper,  thus  providing  fresh  metallic 
solution  as  fast  as  the  original  supply  is  used  up. 

844.  Much  use  is  made  of  the  electrotype  process.     It  has  to  a  certain  ex- 
tent taken  the  place  of  stereotyping  in  the  preparation  of  plates  from  which 
books,  charts,  maps,  &c.,  are  printed.   Copperplates  being  harder  than  those 
of  type-metal,  a  far  greater  number  of  copies  can  be  printed  from  them,  and 
they  are  therefore  preferable  for  works  that  are  likely  to  have  an  extensive 
circulation.    When  the  types  are  set,  a  mould  of  each  page  is  taken  in  wax, 
brushed  over  with  plumbago,  and  subjected  to  the  above  process  till  a  thin 
deposit  is  formed,  which  is  made  of  sufficient  thickness  to  print  from  by  back- 
ing it  with  type-metal.     This  book  is  printed  from  electrotype  plates. 

Engravings  both  on  wood  and  copper  are  reproduced  in  the  same  way, 
their  fine  lines  being  brought  out  with  exquisite  perfection.  The  originals 
are  put  away,  and  the  duplicates  alone  used  in  printing.  By  multiplying 
copies,  which  is  done  with  little  or  no  injury  to  the  face  of  the  original,  any 
number  of  impressions  can  be  obtained. — Fac-similes  of  delicate  leaves,  the 
wings  of  insects,  and  even  daguerreotypes,  may  be  made  in  a  similar  way. 

place  ?  What  sort  of  copies  are  thus  obtained  ?  "What  must  be  done,  to  obtain  fac- 
eimiles  ?  On  what  alone  will  this  metallic  deposit  take  place  ?  How  may  it  be  made 
to  take  place  on  a  bad  conductor  ?  What  precaution  is  necessary,  to  enable  us  to  re- 
move the  copy  from  the  original  ?  843.  Describe  the  most  convenient  mode  of  elec- 
trotyping. 844.  For  what  is  the  electrotype  process  used  ?  In  what  case  are  copper 
plates  preferable  to  those  of  type-metal  ?  State  the  process  gone  through  in  prc- 


328  VOLTAIC   ELECTRICITY. 

845.  Protection  of  Metals. — Voltaic  electricity  has  been 
applied  to  the  protection  of  metallic  surfaces  from  corro- 
sion.    If  a  given  metal  is  acted  on  by  an  acid  or  saline  so- 
lution, we  have  only  to  immerse  in  the  liquid  some  other 
metal  more  readily  acted  on  by  it,  and  close  the  circuit  by 
connecting  the  two,  when  the  chemical  action  on  the  for- 
mer metal  at  once  ceases  and  is  transferred  to  the  latter. 

Davy  proposed  on  this  principle  to  protect  the  copper  sheathing  on  the 
bottom  of  vessels  from  the  action  of  sea-water.  Strips  of  zinc  were  fastened 
at  certain  distances  on  the  copper,  and  it  was  found  that  the  latter  metal  was 
thus  perfectly  preserved  from  corrosion.  No  practical  use,  however,  could 
be  made  of  this  proposed  improvement ;  for  shell-fish,  sea-weed,  Ac.,  which 
had  before  been  kept  off  by  the  poisonous  properties  of  the  corroded  copper, 
now  adhered  to  the  bottom  in  such  quantities  as  to  make  the  vessel  sail  more 
slowly. 

846.  Luminous  and  Heating  Effects. — When  the  gal- 
vanic circle  is  closed  or  broken, — that  is,  when  the  two 
terminal  wires  are  brought   in   contact  or   separated, — a 
bright  spark  passes  between  them.     With  the  proper  ap- 
paratus, this  spark  may  be  intensified  into  the  most  bril- 
liant light  yet  produced  by  art,  known  as  the  Electric 
Light,  or  the  Voltaic  Arch. 

To  produce  the  electric  light,  connect  the  poles  of  a 
powerful  battery  with  the  rods  of  a  universal  discharger 
(§  780),  and  to  the  extremities  of  these  rods  fix  charcoal 
points,  or  pieces  of  graphite  pointed  like  a  pencil.  The 
battery  being  set  in  operation,  the  charcoal  points  are 
brought  in  contact,  and  then  gradually  withdrawn  from 
each  other  a  short  distance,  when  the  space  between  them 
is  spanned  by  an  arch  of  intensely  bright  light. 

The  voltaic  arch  is  widest  in  the  centre ;  its  length  varies  with  the  power 
of  the  battery,  ranging  between  three-fourths  of  an  inch  and  four  inches. 
No  luminous  appearance  is  produced  unless  the  points  first  touch,  no  matter 
how  close  together  they  are  brought,  the  air  between  being  an  insulator  and 

paring  the  plates.  What  else  are  reproduced  by  the  electrotype  process  ?  845.  To 
what  has  voltaic  electricity  been  applied  ?  How  may  a  metal  acted  on  by  a  liquid  in 
which  it  is  immersed  be  protected  from  corrosion  ?  What  application  of  this  princi- 
ple was  proposed  by  Davy  ?  What  was  the  result  of  the  experiment  ?  846.  What 
takes  place  when  the  galvanic  circuit  is  closed  or  broken  ?  Into  what  may  this  spark 
be  intensified  ?  How  is  the  electric  light  produced  ?  What  is  the  shape  of  the  arch. 


THE  ELECTRIC  LIGHT.  329 

breaking  the  circuit.  In  a  vacuum,  however,  the  arch  may  be  formed  with- 
out previous  contact ;  and  even  in  the  air,  if  when  the  points  are  brought 
near  each  other  a  charge  from  a  Ley  den  jar  is  passed  from  one  to  the  other. 

The  electric  light,  like  the  electric  spark,  is  entirely  independent  of  com- 
bustion. None  of  the  carbon  is  consumed,  though  a  portion  of  it  is  mechan- 
ically carried  over  with  a  sort  of  hissing  sound  from  the  positive  to  the 
negative  electrode,  as  is  shown  by  the  change  of  shape  in  the  points  when 
the  experiment  is  over.  The  electric  light  may  be  produced  in  a  vacuum 
and  even  under  water,  which  shows  that  it  is  not  the  result  of  combustion. 

The  intensity  of  the  electric  light  depends  rather  on  the  -size  of  the  me- 
tallic plates  employed  than  on  their  number ;  that  is,  on  the  quantity  of 
electricity  developed  more  than  its  intensity.  The  arch  produced  with  a 
powerful  battery  is  about  one-third  as  intense  as  that  of  the  sun  ;  while  the 
Drummond  light,  which  stands  next  to  it  among  artificial  lights  in  point  of 
brilliancy,  has  only  about  1/1M  of  the  sun's  intensity.  It  has  been  proposed 
to  use  the  electric  light  for  illuminating  the  streets  of  cities ;  but  the  great 
expense  of  maintaining  a  sufficient  voltaic  current  has  thus  far  prevented  its 
introduction  for  that  purpose. 

847.  Heat,  as  well  as  light,  is  produced  in  the  greatest 
intensity  yet  known  to  man  by  the  galvanic  battery.  The 
hardest  substances  introduced  within  the  voltaic  arch,  or 
brought  between  the  electrodes  of  a  powerful  battery  to 
close  the  circuit,  are  instantly  ignited  or  fused.  Platinum, 
which  withstands  the  fiercest  heat  of  the  furnace,  melts  like 
wax  in  the  flame  of  a  candle.  Quartz,  the  precious  stones, 
the  earths,  the  firmest  and  most  refractory  compounds,  are 
fused  in  like  manner.  Thin  leaves  of  metal  subjected  to 
the  action  of  a  battery  burn  with  great  brilliancy  and  beau- 
ty, yielding  flames  of  different  colors.  Gold  and  zinc  burn 
with  a  vivid  white  light,  silver  with  an  emerald  green,  cop- 
per and  tin  with  a  pale  blue,  lead  with  a  brilliant  purple, 
and  steel  watch-spring  with  the  brightest  scintillations. — 
The  heat  produced  by  a  battery,  like  its  light,  depends  on 
the  size  of  the  plates  rather  than  their  number. 

The  heating  power  of  a  galvanic  battery  may  be  shown  by  experiments 
with  wires  of  different  metals  stretched  between  the  electrodes.  A  wire  so 

and  its  length  ?  What  is  essential  to  its  production  in  the  air  ?  Is  this  necessary  in 
a  vacuum  ?  How  is  it  proved  that  the  electric  light  is  not  the  result  of  combustion  ? 
On  what  does  the  intensity  of  the  electric  light  depend  ?  How  does  its  intensity 
compare  with  that  of  the  sun  and  the  Drummond  light?  For  what  has  it  been  pro- 
posed to  use  the  electric  light  ?  847.  What  is  said  of  the  heat  produced  by  the  galvan- 
ic battery  ?  State  some  of  its  effects.  On  what  does  the  heat  produced  by  a  battery 


330  VOLTAIC  ELECTRICITY. 

placed  instantly  becomes  hot ;  if  not  too  long,  red  hot.  By  reducing  its  length, 
we  may  raise  it  to  a  white  heat,  and  by  shortening  it  still  further  we  may 
fuse  or  ignite  it.  Experiments  with  different  metallic  wires  of  the  same  size 
and  length,  show  that  they  are  not  all  heated  to  the  same  degree  by  a  given 
battery.  The  best  conductors  allow  the  current  to  pass  with  the  least  ob- 
struction, and  are  therefore  heated  the  least. 

Platinum  wire  (which  is  one  of  the  poorest  metallic  conductors  and  there- 
fore most  readily  heated),  immersed  in  a  small  quantity  of  water  between  the 
electrodes  of  a  battery,  causes  the  water  to  boil.  Passed  through  phospho- 
rus, ether,  and  ajcohol,  it  ignites  them.  Gunpowder  is  exploded  by  contact 
with  such  a  wire,  a  fact  which  is  turned  to  account  in  the  firing  of  blasts  and 
submarine  batteries.  The  platinum  wire  being  carried  through  the  powder 
and  connected  with  the  positive  and  negative  electrodes,  no  matter  how  far 
off  the  battery  may  be,  the  moment  the  circuit  is  completed  the  platinum  be- 
comes red  hot,  and  the  explosion  takes  place.  By  thus  simultaneously  firing 
a  number  of  charges  of  powder  placed  in  deep  holes  at  certain  distances, 
600,000  tons  of  rock  have  been  instantly  blown  off  from  the  face  of  a  cliff, 
with  an  immense  saving  of  labor,  and  with  perfect  safety  on  'the  part  of  the 
operator,  who  with  his  instrument  was  a  fifth  of  a  mile  from  the  scene  of 
the  blast. 

848.  Physiological  Effects. — The  singular  effects  of  the 
galvanic  fluid  on  the  nerves  and  muscles  of  animals,  origi- 
nally led,  as  we  have  seen,  to  the  development  of  the  sci- 
ence of  Galvanism,  and  were  carefully  investigated  in  the 
earlier  stages  of  its  history.  The  more  powerful  instru- 
ments since  invented  have  enabled  experimenters  to  push 
their  researches  still  further. 

When  we  grasp  the  electrodes  of  a  battery  of  fifty  cups, 
one  in  each  hand,  we  feel  a  peculiar  twinge  in  the  elbow 
and  sometimes  in  the  shoulder,  as  if  the  joints  were  being 
wrenched  apart.  This  sensation  continues  as  long  as  the 
electrodes  are  held  in  the  hands,  and  when  we  first  grasp 
them  or  let  them  go  is  sufficiently  sudden  and  vivid  to  be 
called  a  shock.  A  number  of  persons  may  take  the  shock 
at  once  by  joining  their  hands,  which  should  be  previously 


depend  ?  What  is  the  effect  of  the  galvanic  battery  on  metallic  wires  ?  When  wires 
of  different  metals  are  used,  what  is  found  ?  How  is  this  explained  ?  What  experi- 
ments may  be  performed  with  a  platinum  wire  fixed  between  the  electrodes  of  a  bat- 
tery ?  Describe  the  process  of  firing  a  blast  with  such  a  wire.  What  instance  i» 
mentioned  of  the  practical  application  of  this  process  ?  848.  What  originally  led  to 
the  development  of  galvanism  as  a  science  ?  What  sensation  is  experienced  on  grasp- 
ing the  electrodes  of  a  battery  ?  How  may  a  number  of  persons  take  the  shock  ? 


PHYSIOLOGICAL  EFFECTS.  331 

moistened.  A  weak  current  passed  through  the  eyes  pro- 
duces a  faint  flash  ;  passed  through  the  ears,  a  roaring 
sound ;  and  through  the  tongue,  a  metallic  taste. 

The  effects  of  the  galvanic  battery  on  the  animal  system,  unlike  its  lumi- 
nous and  heating  effects,  are  found  to  depend  on  the  number  of  plates  em- 
ployed rather  than  their  size, — that  is,  on  the  intensity  of  the  electricity  pro- 
duced, and  not  its  quantity.  A  battery  of  several  hundred  pair  of  plates 
proves  fatal  to  life.  One  of  a  hundred  pair  gives  a  shock  that  few  would 
like  to  bear  a  second  time,  though,  if  the  plates  are  small,  it  has  no  effect  on 
wires  stretched  between  the  electrodes.  Put  the  same  amount  of  metallic 
surface  in  a  few  pair  of  very  large  plates,  and  such  a  battery  will  instantly 
fuse  wires  subjected  to  its  action,  while  its  shock  will  hardly  be  felt. 

849.  There  seems  to  be  a  remarkable  analogy  between 
a  voltaic  current  and  the  nervous  energy.    Experiment  has 
shown  that,  if  a  nerve  be  divided,  a  galvanic  current  di- 
rected through  the  region  in  which  it  runs  will  in  a  meas- 
ure supply  its  place.     The  part,  which  would  otherwise  be 
palsied  from  a  want  of  nervous  energy,  may  thus  be  re- 
stored to  its  usual  action.     If,  for  example,  the  nerves  of 
the  stomach  are  divided,  digestion  ceases ;  but  it  is  resumed 
if  the  stomach  is  subjected  to  galvanic  influence.     Galvan- 
ism is  therefore  medically  applied  in  asthma,  paralysis,  and 
other  diseases  arising  from  a  prostration  of  the  nervous 
system. 

850.  Among  the  most  remarkable  effects  of  voltaic  elec- 
tricity are  the  violent  contortions  it  produces  in  bodies  just 
deprived  of  life. 

A  few  years  ago,  the  body  of  a  murderer  hanged  in  Glasgow  was  sub- 
jected, about  an  hour  and  a  quarter  after  his  execution,  to  the  action  of  a 
battery  consisting  of  270  pair  of  four-inch  plates.  One  pole  was  applied  to 
the  spinal  marrow  at  the  nape  of  the  neck,  and  the  other  to  the  sciatic  nerve 
in  the  left  hip,  when  the  whole  body  was  thrown  into  a  violent  tremor  as  if 
shivering  with  cold.  On  removing  the  wire  from  the  sciatic  nerve  to  a  nerve 
in  the  heel,  the  leg  was  thrown  out  so  violently  as  nearly  to  overturn  one  of 


What  is  the  effect  of  passing  a  weak  current  through  the  eyes  ?  Through  the  ears? 
Through  the  tongue  ?  On  what  do  the  effects  of  the  galvanic  battery  on  the  animal 
system  depend  ?  Compare  the  different  effects  of  a  given  amount  of  metallic  surface, 
when  thrown  into  many  small  plates,  and  a  few  large  ones.  849.  To  what  does  the 
voltaic  current  bear  a  remarkable  analogy  ?  What  has  been  shown  by  experiment  ? 
Give  an  example.  In  what  diseases  is  galvanism  medically  applied  ?  850.  What  is 
one  of  the  most  remarkable  effects  of  voltaic  electricity?  Describe  the  experiment* 


332  THERMO-ELECTRICITY. 

the  assistants,  who  tried  in  vain  to  prevent  its  extension.  On  directing  a 
current  to  the  principal  muscle  of  respiration,  the  chest  heaved  and  fell,  and 
labored  breathing  commenced.  When  one  of  the  poles  was  applied  to  a 
nerve  under  the  eyebrow  and  the  other  to  the  heel,  the  most  extraordinary 
grimaces  were  produced :  "  every  muscle  of  the  countenance  was  simulta- 
neously thrown  into  fearful  action;  rage,  horror,  despair,  anguish,  and 
ghastly  smiles,  united  their  hideous  expression  in  the  murderer's  face."  Sev- 
eral spectators  were  so  overcome  by  the  sight  that  they  had  to  leave  the 
room,  and  one  gentleman  fainted.  In  the  last  experiment,  the  fore  finger, 
which  had  previously  been  bent,  was  instantly  extended,  and  shaking  vio- 
lently, with  a  convulsive  movement  of  the  whole  arm,  seemed  to  point  to  the 
persons  present,  some  of  whom  thought  that  the  body  had  really  returned 
to  life. 

Thermo-electricity ; 

OR,    ELECTRICITY   DEVELOPED   BY   HEAT. 

851.  How  PRODUCED. — If  two  strips  of  metals  which 
differ  in  their  conducting  power,  are  soldered  together  at 
one  end  so  as  to  form  an  acute  angle  with  each  other,  and 
heat  is  applied  at  the  place  of  junction,  a  current  of  elec- 
tricity is  produced,  which  may  be  carried  off  by  any  good 
conductor.  Antimony  and  bismuth  exhibit  this  phenome- 
non in  its  greatest  perfection,  and  are  generally  used  in 
performing  the  experiment.  Electricity  thus  developed  by 
heat  is  known  as  Thermo-electricity.  Its  properties  are  the 
same  as  those  of  frictional  electricity. 

Fig.  sot.  852.    THERMO-ELECTRIC    BATTERIES. 

5666  — Thermo-electricity  may  be  developed 

\l\/\/\f\fl      abundantly  by  combining  a  number  of 

\j  Ml  \U  \J  Ml        thin  bars  of  antimony  and  bismuth,  or 

a    a   a    a    <          platinum  and  iron.     They  may  be  ar- 

666          ranged  in  either  of  the  forms  represent- 

1    ed  in  Fig.  301,  or  may  be  laid  flat  one 

L=!J    fc=U    I=U   I==!J    upon  another,  with  pasteboard  between 

to  prevejit  them  from  touching  except 

THERMO-ELECTRIC    BAT-  *  .  .    .  .  * 

TEBIES.  at  their  extremities.     By  heating  the 

points  of  junction  at  one  end,  «,  a,  a,  a,  and  cooling  those 

performed  on  the  body  of  a  murderer  shortly  after  his  execution.  851.  What  iff 
Thermo-electricity  ?  How  is  it  produced  ?  What  metals  are  generally  used  in  pro- 
ducing it  ?  852.  How  may  thermo-electricity  be  developed  abundantly  ?  How  is  a 


THERMO-ELECTRIC  BATTEEIES.  333 

at  the  other,  #,  5,  #,  5,  an  electric  current  is  produced,  the 
intensity  of  which  is  equal  to  the  sum  of  the  intensities  of 
the  separate  pairs.  With  a  wire  attached  to  the  first  bar 
of  bismuth  and  another  attached  to  the  last  bar  of  anti- 
mony, the  thermo-electric  current  may  be  conducted  wher- 
ever it  is  desired. 

When  thirty  or  forty  such  combinations  are  needed,  thin  metallic  bars 
are  used,  connected  alternately  at  their  extremities,  and  arranged  for  conve- 
nience' sake  in  parallel  piles  of  five  or  six  each.  Such  a  battery  indicates 
changes  of  temperature  at  its  junctions  so  minute  that  they  can  be  detected 
in  no  other  way, — even  to  the  hundredth  part  of  a  degree  of  the  thermometer. 
The  heat  radiated  from  the  hand  is  sufficient  to  produce  a  slight  electric 
current. 

853.  Electricity,  besides  being  produced  by  friction, 
chemical  action,  and  heat,  is  also  developed  under  certain 
conditions  by  magnetism.  When  so  produced,  it  is  called 
Magneto-electricity.  This  branch  of  the  subject  can  not 
be  understood  till  we  have  treated  of  Magnetism,  and  will 
therefore  be  considered  in  the  next  chapter,  which  is  de- 
voted to  that  subject. 


CHAPTER   XVII. 
MAGJ-NETISM. 

854.  A  MAGNET  is  a  body  which  has  the  property  of 
attracting  iron  and  being  attracted  by  it. 

=  855.  Magnetism  is  the  science  that  treats  of  the  laws, 
properties,  and  phenomena  of  magnets. 

Kinds  of  Magnets. 

856.  There  are  two  kinds  of  magnets,  Natural  and  Ar- 
tificial. 

thermo-electric  battery  formed  ?  "What  is  the  usual  arrangement  when  a  large  num- 
ber of  such  combinations  are  needed  ?  How  minute  changes  of  temperature  are  indi- 
cated with  such  a  battery  ?  853.  By  what  other  agency  is  electricity  also  developed  ? 
What  is  it  then  called? 


334  MAGNETISM. 

857.  NATURAL  MAGNETS. — The  natural  magnet,  or  load- 
stone, is  an  ore  of  iron,  found  in  great  quantities  in  differ- 
ent parts  of  the  earth,  which  has  the  property  of  drawing 
to  itself  steel  filings,  needles,  or  small  pieces  of  unmagnetic 
iron.  Its  texture  is  hard,  and  its  color  varies  from  reddish- 
brown  to  grey.  Besides  the  loadstone,  nickel,  cobalt,  and 
brass  when  hammered  are  found  to  have  magnetic  proper- 
ties, though  in  an  inferior  degree. 

858.  The  attraction  of  the  loadstone  for  particles  of  iron  appears  to  have 
been  known  to  the  Greeks,  Chinese,  and  other  nations  in  remote  antiquity. 
It  is  distinctly  alluded  to  by  Homer  and  Aristotle.  Pliny  speaks  of  a  chain 
of  iron  rings  suspended  one  from  another,  the  first  of  which  was  upheld  by 
a  loadstone.  He  tells  us,  also,  that  Ptolemy  Philadelphus  proposed  to  build 
a  temple  at  Alexandria,  the  ceiling  of  which  was  to  be  of  loadstone,  that  its 
attraction  might  hold  an  iron  statue  of  his  queen  Ar-sin'-o-e  suspended  in  the 
air.  Death  prevented  Ptolemy  from  carrying  out  his  design ;  but  St.  Au- 
gustine, at  a  later  day,  mentions  a  statue  thus  actually  held  in  suspension  in 
the  temple  of  Ser^-a-pis,  at  Alexandria. — The  magnet  (magnes  in  Greek)  is 
supposed  to  have  received  its  name  from  Magnesia,  a  city  9f  Asia  Minor,  near 
which  it  was  first  found. 

859.  Poles. — The  attractive  power  of  a  natural  magnet 
does  not  reside  equally  in  all  its  parts,  but  is  strongest  at 
its  extremities  and  diminishes  towards  the  middle,  where 
it  is  entirely  wanting.  This  is  shown  by  rolling  a  piece  of 
loadstone  in  iron  filings.  They  will  be  found  to  cluster 
about  the  ends,  those  that  first  adhere  being  endowed  with 
the  power  of  attracting  others,  till  large  tufts  are  formed, 
while  the  middle  is  left  entirely  bare. 

The  points  at  which  the  greatest  attractive  power  is 
exhibited,  are  called  the  Poles  of  the  magnet.  The  central 
part,  where  it  is  wanting,  is  called  the  Neutral  Line. 

If  a  piece  of  loadstone  is  broken,  each  portion  becomes 
a  perfect  magnet,  and  has  poles  of  its  own. 

854.  What  is  a  Magnet?  855.  What  is  Magnetism?  856.  How  many  kinds  of 
magnets  are  there  ?  Name  them.  857.  What  is  the  natural  magnet  ?  What  other 
metals  have  magnetic  properties  ?  858.  To  whom  and  when  was  the  attraction  of 
loadstone  for  iron  known  ?  What  ancient  authors  allude  to  it  ?  Of  what  does  Pliny 
speak?  What  use  did  Ptolemy  Philadelphus  propose  to  make  of  the  loadstone? 
What  is  mentioned  by  St.  Augustine  ?  From  what  did  the  magnet  receive  its  name  ? 
859.  What  is  shown  by  rolling  a  piece  of  loadstone  in  iron  filings  ?  What  is  meant 
by  the  Poles  of  the  magnet  ?  What  is  the  Neutral  Line  ?  If  a  piece  of  loadstone  is 


NATURAL  MAGNETS. 


335 


Fig.  302. 


860.  Power  of  Natural  Magnets. — When  quite  small,  a 
natural  magnet  will  sustain  many  times  its  own  weight  of 
iron.     Sir  Isaac  Newton  is  said  to  have  worn,  in  a  ring,  a 
piece  of  loadstone  weighing  three  grains,  which  would  lift 
750  grains  of  iron.     Their  attractive  power,  however,  does 
not  increase  with  their  size.     Large  pieces  of  loadstone 
never  support  more  than  five  or  six  times  their  own  weight, 
and  rarely  as  much.     The  most  powerful  natural  magnet 
known  is  capable  of  lifting  310  pounds. 

861.  Armature. — The  power  of 
a  natural  magnet  is  increased  by 
applying  vertically  to  its  opposite 
polar  surfaces  thin  strips  of  soil 
iron,  projecting  a  little  below,  and 
bent,  as  shown  in   ap,  bn,  Fig. 
302.      The   attractive   force   then 
centres  in  p  and  ?i,  which  become 

the  new  poles.  This  arrangement  is  called  an  Armature, 
and  a  magnet  so  prepared  is  said  to  be  armed. 

To  keep  the  armature  in  its  place,  metallic 
bands,  A  B,  C  D  (Fig.  303),  are  passed  round  the 
whole.  A  ring,  R,  is  attached  to  the  top  for  con- 
venience of  handling.  The  effect  of  the  magnet 
is  further  increased  by  uniting  its  poles  with  a 
transverse  piece  of  soft  iron,  K,  called  the  Keeper. 
To  this  a  hook  is  attached  for  suspending  a  scale- 
pan  and  weights. 

862.  ARTIFICIAL  MAGNETS. — A  piece 
of  iron  or  steel  brought  in  contact  with 
a  natural  magnet  or  very  near  it,  ac- 
quires its  peculiar  properties,  and  will 
itself  attract  steel-filings,  needles,  &c. 
Soft  iron  loses  these  properties  on  be- 
ing withdrawn  from  the  magnet ;  but  a  piece  of  steel  re- 
tains them  permanently,  nor  does  the  natural  magnet  from 

'broken,  what  is  said  of  the  fragments?  860.  "What  is  the  power  of  natural  magnets, 
when  small  ?  When  large  ?  How  much  is  the  most  powerful  natural  magnet  known 
capable  of  lifting  ?  861.  How  is  the  power  of  a  natural  magnet  increased  ?  Describe 
the  armature  and  the  arrangement  for  securing  it  in  its  place.  How  is  the  effect  of 


Fig.  303. 


ABMED  MAGNET. 


336 


MAGNETISM. 


which  it  receives  them  suffer  any  diminution  of  power  in 
consequence. 

A  piece  of  iron  or  steel  to  which  magnetic  properties 
have  been  imparted  in  any  way,  is  called  an  Artificial 
Magnet. 

863.  Kinds  of  Artificial  Magnets. — There  are  several 
kinds  of  artificial  magnets,  called  from  their  shape  Bar  Mag- 
nets, Horse-shoe  Magnets,  and  Magnetic  Needles.  The  first 
two  are  most  powerful  when  formed  of  several  similar  pieces 
riveted  together,  in  which  case  they  are  called  Compound 
Magnets. 

Fig.  304.  Fig.  805. 


COMPOUND    BAB   MAGNET. 

Fig.  304  represents  a  Compound  Bar  Magnet ;  Fig.  305, 
a  Compound  Horse-shoe  Magnet.  N,  S,  represent  the  poles.' 
The  Horse-shoe  magnet  has  an  armature,  A,  attached,  which 
increases  and  preserves  its  power,  and  should  always  be 
kept  on  when  the  magnet  is  not  in  use. 

Magnetic  Needles  are  very  light  magnetic 
bars  (see  Fig.  306),  poised  at  their  centre  on  a 
pivot,  on  which  they  move  freely  either  hori- 


IIOR8E-SHOE 
MAGNET. 


Fig.  306. 


MAGNETIC   NEEDLE. 


zontally  or  up  and  down. 
In  the  former  case,  they 
are  called  Horizontal  Nee- 
dles; in  the  latter,  Verti- 
cal or  Dipping  Needles. 

864.  Artificial  magnets  are  more 
efficient  and  regular  in  their  action 
than  natural  ones,  and  are  therefore 
preferred  for  purposes  of  experi- 
ment. The  horse-shoe  is  more 


the  magnet  further  increased?  862.  How  may  magnetic  properties  be  imparted  to  a 
piece  of  iron  or  steel?  What  is  the  difference  between  soft  iron  and  steel  in  this 
eonnection  ?  What  is  an  Artificial  Magnet  ?  863.  Name  the  different  kinds  of  artifi- 
cial magnets.  What  are  Compound  Magnets?  What  do  Figs.  304,  305,  represent  ? 
What  are  Magnetic  Needles ?  Into  what  two  classes  are  they  divided?  864.  How 
do  artificial  magnets  compare  in  efficiency  with  natural  ones  ?  How  does  the  horse- 


PEOPEETIES  OP  THE  MAGNET.  337 

powerfufthan  the  bar  magnet.  A  horse-shoe  of  one  pound  has  been  known 
to  sustain  26  Va  pounds. 

865.  Poles. — The  poles  of  an  artificial  magnet, — that  is, 
the  points  in  which  the  greatest  attractive  force  resides, — 
are  found  to  be  about  one-tenth  of  an  inch  from  the  ex- 
tremities.    In  very  long  bar  magnets,  besides  the  two  poles 
always  situated  near  the  extremities,  two  other  poles,  nearer 
the  centre,  are  sometimes,  though  rarely,  found. 

866.  The  power  of  a  magnet,  whether  natural  or  artifi- 
cial, may  be  increased  by  daily  adding  a  little  to  the  weight 
which  it  will  support.     If,  for  instance,  a  given  magnet  just 
sustains  two  pounds  of  iron,  by  putting  on  a  small  addi- 
tional weight  every  day,  we  may  perhaps  make  it  sustain 
three  or  even  four  pounds.     If,  on  the  other  hand,  we  over- 
load it,  so  that  the  armature  falls  off,  the  power  of  the  mag- 
net will  be  impaired.     Any  rough  treatment,  such  as  ham- 
mering the  magnet,  rubbing  it  violently,  or  letting  it  fall, 
has  the  same  effect.     Heat,  also,  diminishes  the  power  of  a 
magnet.     Red  heat  destroys  it  altogether,  even  after  the 
magnet  has  cooled. 

867.  Air  is  not  essential  to  the  action  of  a  magnet;  all 
its  phenomena  may  be  exhibited  in  a  vacuum. 

Properties  of  the  Magnet. 

868.  ATTRACTION. — As  stated  above,  all  magnets  attract 
unmagnetic  iron.     They  are  also  attracted  by  it. 

Suspend  a  magnetic  needle  by  a  thread.    Bring  a  piece.of  iron  near  either 
extremity,  and  the  needle  will  be  drawn  towards  it. 

869.  Magnetic  attraction  acts  with  undiminished  power 
through  any  thin  substance. 

In  the  last  experiment  interpose  a  piece  of  glass  or  paste-board  between 
the  iron  and  the  needle ;  the  latter  will  be  attracted  none  the  less. 

•hoe  compare  with  the  bar  magnet  f  865.  Where  do  the  poles  of  an  artificial  magnet 
lie  ?  What  are  sometimes  fonnd  in  very  long  bar  magnets  ?  866.  What  is  the  effect 
of  adding  a  little  daily  to  the  weight  which  a  magnet  supports  ?  Give  an  example. 
What  is  the  effect  of  overloading  a  magnet  ?  Of  treating  it  roughly  ?  Of  heating  it  ? 
86T.  Is  air  essential  to  the  action  of  a  magnet  ?  Prove  it  868.  What  is  the  first  prop- 
erty of  magnets  ?  What  experiment  shows  the  attraction  of  iron  for  a  magnet  f 
869.  What  is  the  effect  of  interposing  any  thin  substance  ?  How  may  this  fact  b» 

15 


MAGNETISM. 


MAGNETIC  CUBVE9. 


Tig.  307.  Hold  a  piece  of  paper  over  4 

bar  magnet,  and  dust  on  it  some 
steel  filings.  Under  the  influence 
of  the  magnetic  attraction  trans- 
mitted through  the  paper,  they 
will  arrange  themselves  in  regu- 
lar lines,  as  shown  in  Fig.  307. 
These  lines  are  called  Magnetic 
Curves. — The  superior  attractive 
power  of  the  poles  is  also  shown 
by  this  experiment ;  for  the  filings  are  thickest  directly  over  those  points,  the 
curves  appearing  to  converge  there  from  all  directions. 

Magnetic  figures  of  any  description  may  be  formed  on  a  steel  plate  by 
marking  on  it  with  one  of  the  poles  of  a  bar  magnet,  and  then  sprinkling 
iron  filings  on  the  surface.  They  will  at  once  adhere  to  the  lines  which  the 
magnet  has  traced.  The  result  is  the  same  if  paper  is  laid  on  the  steel  sur- 
face before  the  bar  is  drawn  over  it,  the  magnetic  influence  being  transmitted 
through  the  paper. 

870.  Law. — Magnetic  attraction  decreases  in  intensity 
as  the  square  of  the  distance  from  the  magnet  increases. 

If  two  similar  substances  are  situated  respectively  1  inch  and  2  inches 
from  a  given  magnet,  the  former  will  be  attracted  4  times  as  strongly  as  the 
latter.  This  law  corresponds  with  that  of  gravitation,  light,  and  heat. 

871.  POLARITY. — A  magnetic  needle,  left  free  to  move, 
always  points  north  and  south,  or  nearly  so.     Often  as  it 
may  be  disturbed  from  its  natural  position,  it  invariably  re- 
sumes it  after  a  few  vibrations.     This  property  is  called 
Magnetic  or  Directive  Polarity. 

It  is  to  be  observed  in  connection  with  magnetic  polarity 
that  the  same  extremity  of  the  needle  always  points  to  the 
north,  and  the  same  extremity  to  the  south.  That  which 
points  north  is  called  the  North  Pole  ;  and  that  which  points 
south,  the  South  Pole.  Turn  the  needle  round  till  its  north 
pole  points  south,  and  it  will  not  rest  till  it  has  traversed 
a  semicircle  and  got  round  again  to  the  north. 

872.  If  the  poles  of  a  bar  or  horse-shoe  magnet  be  pre- 
sented successively  to  the  north  pole  of  a  magnetic  needle, 

illustrated  ?  How  are  Magnetic  Carves  formed  ?  What  does  this  experiment  show  ? 
How  may  magnetic  figures  be  formed  ?  What  is  the  effect  of  Interposing  paper  be- 
tween the  magnet  and  the  steel  surface  ?  870.  What  is  the  law  of  magnetic  attrac- 
tion ?  Give  an  example.  871.  What  is  meant  by  Magnetic  or  Directive  Polarity  ? 
What  is  to  be  observed  in  connection  with  magnetic  polarity  1  What  name  is  given 


MAGNETIC  POLAEITY.  339 

one  of  them  will  be  found:  to  attract  it  and  the  other  to 
repel  it.  If  the  experiment  be  tried  with  a  number  of  dif- 
ferent needles,  the  same  pole  will  always  be  found  to  at- 
tract, and  the  same  to  repel.  This  shows  that  the  two  poles 
of  the  magnet  have  different  properties,  which  we  indicate 
by  giving  them  different  names.  The  one  that  attracts  the 
north  pole  of  the  needle  we  call  the  South  Pole  of  the 
magnet,  and  the  one  that  repels  it,  the  North  Pole. 

873.  General  Law. — Like  poles  of  magnets  repel  each, 
other,  and  unlike  poles  attract  each  other.    This  law  corre- 
sponds with  that  of  electrical  attraction  and  repulsion. 

Balance  a  bar  magnet  with  weights  on  a  pair  of  scales.  Beneath  its  pos- 
itive pole  bring  the  positive  pole  of  another  magnet,  and  the  scale  containing 
the  bar  will  rise  owing  to  the  repulsion  of  the  like  poles.  Substitute  the  neg- 
ative pole,  and  the  scale  will  descend  owing  to  the  attraction  of  the  unlike  poles. 

874.  Like  poles  neutralize  each  other's  attraction  for 
unmagnetic  iron. 

Immerse  the  positive  poles  of  two  magnets  separately  in  iron  filings.  On 
withdrawing  them,  both  will  be  covered  with  large  tufts.  Now  bring  them 
together,  and  the  filings  will  immediately  drop  off  from  both.  The  result 
will  be  the  same  if  the  experiment  be  tried  with  the  negative  poles  of  two 
magnets.  If  the  positive  pole  of  one  magnet  and  the  negative  of  the  other 
be  used,  the  filings,  instead  of  falling  off,  will  join  in  a  festoon  between  the 
two  unlike  poles. 

875.  The  Astatic  Needle.— The 
polarity  of  two  needles   of  equal 
power  may  be  neutralized  by  sup- 
porting them  on  the  same  pivot,  one 
above  the  other,  parallel  and  with 
unlike  poles  pointing  in  the  same 
direction.  An  instrument  so  formed 
is  called  the  Astatic  Needle. 

Fig.  308  represents  an  astatic  needle.  The 
north  pole  of  the  upper  one  points  the  same 
way  as  the  south  pole  of  the  under  one,  and  ASTATIC  NKEDLB. 

to  the  two  poles  of  the  needle  ?  872.  How  is  it  shown  that  the  poles  of  a  bar  magnet 
have  different  properties  ?  How  are  these  poles  distinguished  ?  873.  What  is  the 
law  of  magnetic  attraction  and  repulsion  ?  Illustrate  this  law  with  an  experiment. 
874.  What  is  the  effect  of  like  poles  on  each  other's  attraction  ?  Show  this  experi- 
mentally. 875.  How  may  the  polarity  of  two  needles  of  equal  power  be  destroyed? 


340  '  MAGNETISM. 

vice  versa.  The  consequence  is  that  the  polarity  of  both  is  destroyed ;  the 
needles  will  remain  in  whatever  direction  they  are  placed. 

876.  When  a  magnet  is  divided,  each  portion  becomes 
a  perfect  magnet  in  itself,  and  has  its  own  poles,  even  though 
the  parts  in  which  the  new  poles  lie  exhibited  no  magnetic 
attraction  at  all  before  the  division.  Those  extremities  of 
the  divided  portions  which  lie  towards  the  north  pole  of 
the  original  magnet  will  all  be  north  poles,  and  the  extrem- 
ities towards  its  south  pole  will  all  be  south  poles. 

8V7.  Magnetic  Variation. — In  a  given  place,  all  mag- 
netic needles  point  in  the  same  direction.  This  direction 
is  called  the  Magnetic  Meridian. 

•  In  some  parts  of  the  earth  the  magnetic  meridian  runs 
due  north  and  south ;  that  is,  a  plane  extended  in  the  di- 
rection in  which  the  needle  stands  would  pass  through  the 
north  and  the  south  pole  of  the  earth.  The  magnetic  me- 
ridian would  then  correspond  with  the  geographical  me- 
ridian. In  most  places,  however,  the  magnetic  meridian 
deviates  more  or  less  from  the  geographical  meridian.  This 
deviation  is  called  the  Variation  of  the  Needle,  or  Magnetic 
Variation. 

The  variation  of  the  needle  is  different  at  different  places  on  the  earth's 
surface,  and  is  constantly  changing  at  the  same  place.  Recorded  observa- 
tions in  the  old  world  show  that  for  a  series  of  years  the  needle  kept  varying 
more  and  more  towards  the  west ;  till,  having  attained  its  western  limit,  it 
turned  back  towards  the  east,  in  which  direction  it  is  now  moving.  The 
cause  of  this  periodical  change  and  the  law  which  regulates  it  are  as  yet  un- 
known. At  Washington  City  the  variation  is  now  between  2  and  3  degrees 
west;  that  is,  the  needle  points  between  2  and  3  degrees  west  of  north. 
Every  year  it  becomes  somewhat  greater,  the  annual  rate  of  increase  being 
about  3'. 

Two  irregular  lines  (which  are  constantly  changing)  may  be  traced  on 
the  earth's  surface,  one  in  each  hemisphere,  along  which  the  needle  points 
due  north  and  south.  They  are  called  Lines  of  no  Variation. 

8*78.  Magnetic  Dip. — An  ordinary  steel  needle,  poised 

Describe  the  Astatic  Needle.  876.  When  a  magnet  is  divided,  what  is  said  of  each 
portion  ?  Which  extremities  of  the  divided  portions  will  be  north  poles,  and  which 
south  ?  87T.  What  is  the  Magnetic  Meridian  ?  In  some  parts  of  the  earth  how  does 
the  magnetic  meridian  run  ?  How,  in  others  ?  What  is  meant  by  Magnetic  Varia- 
tion? What  do  recorded  observations  show?  What  is  the  present  variation  at 
Washington  City,  and  how  is  it  changing  from  year  to  year?  What  is  meant  by 


MAGNETIC  DIP. 


341 


Fig.  809. 


on  its  centre  of  gravity  so  as  to  move  freely  up  and  down, 
remains  in  any  position  in  which  it  may  be  placed  ;  if  mag- 
netized, in  most  parts  of  the  earth  it  inclines  more  or  less 
towards  the  horizon.  This  inclination  is  called  th#  Dip  of 
the  Needle,  or  Magnetic  Dip.  It  was  discovered  in  1576, 
by  an  optician  of  London. 

With  the  Dipping  Needle  and  graduat- 
ed scale  attached,  represented  in  Fig.  309, 
the  magnetic  dip  at  any  given  place  can  be 
measured.  Experiments  with  this  instru- 
ment show  that  there  are  two  points,  one 
in  the  northern  hemisphere  (latitude  70), 
the  other  in  the  southern  (lat.  75),  in 
which  the  needle  stands  vertical,  and  the 
dip  is  therefore  90  degrees.  That,  on  the 
contrary,  there  is  a  circle  of  poihts  near 
the  equator,  at  which  the  needle  is  par- 
allel to  the  horizon,  and  the  dip  is  0; 
this  line  is  called  the  Magnetic  Equator. 
At  different  intermediate  points  the  dip  is 
different,  increasing,  though  not  regular- 


THE  DIPPING  NEEDLE. 


ly,  as  the  distance  from  the  magnetic  equator  increases.  The  dip,  like  the 
variation,  keeps  changing  at  a  given  place.  In  the  latitude  of  New  York  it 
is  now  about  70  degrees,  and  is  constantly  decreasing. 

879.  The  Compass. — The  polarity  of  the  magnetic  nee- 
dle, applied  in  the  Compass,  enables  us  to  determine,  at 
any  place,  a  given  direction  or  the  bearing  of  a  given  object. 

The  Land  or  Surveyor's  Compass  is  simply  a  magnetic 
needle  set  in  a  shallow  case  covered  with  glass,  on  the  bot- 
tom of  which  is  a  circular  card,  having  its  circumference 
divided  into  360  degrees.  At  a  distance  of  one-fourth  of 
the  circumference  apart  stand  the  letters  N,  E,  S,  W,  de- 
noting the  four  cardinal  points — North,  East,  South,  West. 
As  the  needle  is  stationary,  while  the  card  moves,  the  order 
of  the  points  is  reversed ;  that  is,  when  we  hold  the  instru- 
ment so  as  to  have  the  point  S  next  to  us,  E  is  on  the  left, 
and  W  on  the  right. 

Lines  of  no  Variation  ?  How  many  are  there  ?  878.  What  is  the  Dip  of  the  Needle  f 
When  and  by  whom  was  it  discovered  ?  How  may  the  dip  at  any  given  place  be 
measured  ?  What  is  shown  by  experiments  with  the-  dipping  needle  ?  How  great 
b  the  dip  in  (he  latitude  of  New  York  ?  879.  In  what  instrument  is  the  polarity  of 


342 


MAGNETISM. 


880.  It  is  to  the  navigator,  who  relies  entirely  on  it  for 
guidance  over  the  trackless  ocean  to  his  desired  port,  that 
the  compass  is  most  important.  Arranged  for  his  use,  it  is 
called  the  Mariner's  Compass. 

Fig.  810.  In  the  mariner's 

compass,  repre- 
sented in  Fig.  310, 
the  circular  card 
is  attached  to  the 
needle  and  turns 
with  it.  The  cir- 
cumference of  the 
card  is  divided  into 
32  equal  parts,  de- 
noted by  marks  and 
sometimes  subdi- 
vided into  halves 
and  quarters.  These 
marks  have  names 
given  to  them,  in- 
dicating the  dif- 
ferent directions, 
which  are  called 
Points  of  the  Com- 
pass. Mentioning 

THE  MABINKB'S   COMPASS.  tae    pomtg     Of    the 

•ompass  in  their  order  is  called  boxing  the  compass. — The  compass  box  ii 
suspended  within  a  larger  box  by  means  of  two  brass  hoops,  or  gimbals  at 
they  are  called,  supported  at  opposite  points  on  pivots,  so  that  however  the) 
vessel  may  roll  or  pitch  the  needle  may  retain  its  horizontal  position. 

It  is  believed  that  the  Chinese  were  the  first  to  avail  themselves  of  the 
magnet  in  navigation,  many  hundred  years  before  the  Christian  era ;  and 
that  from  them  various  other  eastern  nations  learned  to  use  it  for  the  same 
purpose.  The  compass  of  these  early  times  was  probably  nothing  more  than 
a  piece  of  loadstone  mounted  on  a  cork  and  allowed  to  float  on  water.  The 
magnetic  needle  and  the  card  attached  to  it  were  no  doubt  the  inventions  of 
Europeans,  among  whom  a  knowledge  of  the  rude  compass  used  in  the  East 
appears  to  have  been  introduced  in  the  twelfth  century  after  Christ.  Flavio 
Gioia  \Jlah '-ve-o  jo '-yaK\,  a  Neapolitan  who  flourished  about  the  year  1300, 

the  magnetic  needle  applied  ?  Describe  the  Land  Compass.  880.  To  whom  is  tho 
compass  most  important?  Describe  the  Mariner's  Compass.  "What  is  meant  by 
boxing  the  compass  f  How  is  the  compass  box  suspended  ?  "Who  are  thought  tq 
have  been  the  first  to  use  the  magnet  in  navigation  ?  "What  did  this  ancient  compass 
probably  consist  of?  "When  did  it  first  become  known  in  Europe  ?  What  improve- 
ments were  soon  made  ?  How  did  the  name  of  Flavio  Gioia  become  connected  with 


THE   COMPASS.  843 

by  seme  regarded  as  the  inventor  of  the  compass,  probably  merely  improved 
its  construction,  or  extended  its  use  among  the  maritime  nations  of  Europe. 

No  one  can  estimate  how  much  the  invention  of  the  mariner's  compass 
has  contributed  to  the  progress  of  the  world.  Relying  on  his  little  needle, 
which  never  betrays  its  trust,  the  mariner  is  no  longer  obliged  to  keep  his 
bark  within  sight  of  land,  and  to  direct  his  course  by  sun  and  star  which 
clouds  may  obscure  for  days  and  nights  together.  He  fearlessly  ventures 
Into  unknown  seas,  explores  the  remotest  regions,  pursues  his  way  under 
lowering  skies  and  in  utter  darkness,  well  knowing  whither  he  is  sailing  and 
how  to  steer  when  he  wishes  to  retrace  his  course.  This  simple  instrument 
has  thus  made  the  ocean  a  safe  and  frequented  highway,  extended  the  com- 
merce and  knowledge  of  the  world,  linked  its  most  distant  families  in  friendly 
intercourse,  and  brought  whole  continents  virtually  into  being. 

881.  The  compass  needle,  like  all  other  magnetic  nee- 
dles, is  subject  to  variation  and  dip. 

Its  variation  seems  to  have  been  known  two  hundred  years  before  the 
time  of  Columbus ;  but  that  this  variation  differs  in  different  places  was  dis- 
covered by  that  navigator  on  his  memorable  voyage  across  the  Atlantic  in 
1492.  As  he  went  westward,  he  observed  that  the  variation  increased  from 
day  to  day.  The  fact  was  soon  discovered  by  his  crew,  and  filled  them  with 
consternation.  It  seemed  '  as  if  the  very  laws  of  nature  were  changing,  and 
they  were  entering  a  new  world  subject  to  mysterious  influences'.  It  re- 
quired all  the  ingenuity  of  Columbus  to  induce  them  to  proceed ;  which  he 
did  by  allaying  their  fears  with  an  explanation  of  the  phenomenon  satisfac- 
tory to  them,  though  it  was  far  from  satisfying  himself. 

As  the  compass  needle  must  be  perfectly  horizontal,  the  dip  is  counter- 
balanced  by  loading  the  end  that  tends  to  rise  with  a  small  weight,  which 
may  be  shifted  to  suit  any  latitude. 


Theory  of  Magnetism. 

882.  The  theory  of  magnetism  is  analogous  to  that  of 
electricity.  An  agent,  which  for  convenience'  sake  we  call 
the  magnetic  fluid,  may  be  supposed  to  pervade  all  things. 
In  its  quiescent  state  it  is  a  combination  of  two  fluids,  which 
may  be  distinguished  as  North  or  Positive,  and  South  or 
Negative.  When  combined,  these  fluids  neutralize  each 
other,  and  no  magnetic  phenomena  are  exhibited;  but 

the  compass?  What  is  said  of  the  effects  which  this  simple  instrument  has  wrought? 
881.  To  what  is  the  compass  needle  subject?  How  long  ago  was  the.  variation  of  the 
needle  known  ?  What  discovery  did  Columbus  make  respecting  it  ?  What  was  the 
effect  of  this  discovery  on  his  crew  ?  How  is  the  dip  counterbalanced  in  the  com- 
pass needle  ?  882.  State  the  theory  of  magnetism.  How  are  the  phenomena  exhib- 


344  MAGNETISM. 

when  through  any  agency  they  are  separated,  the  substance 
containing  them  displays  magnetic  properties,  and  is  said 
to  be  magnetized. 

In  loadstone,  as  found  in  nature,  the  two  fluids  do  not 
combine  at  all.  In  soft  iron  and  steel  they  are  easily  sep- 
arated, but  in  the  former  re-combine  as  soon  as  the  separ- 
ating agency  is  withdrawn,  while  in  the  latter  they  remain 
permanently  apart.  In  most  substances  they  are  united  so 
strongly  as  to  be  almost  incapable  of  separation,  and  such 
substances  are  magnetized  with  the  greatest  difficulty. 

When  the  magnetic  equilibrium  is  disturbed,  and  the  two  fluids  are  sep- 
arated as  described  above,  they  seem  to  take  up  their  abode  in  opposite  sides 
of  the  individual  particles  of  the  magnetized  body,  the  positive  fluid  taking 
the  same  side  throughout,  so  that  the  positive  pole  of  one  particle  is  contigu- 
ous to  the  negative  pole  of  the  next.  Both  fluids  remain  in  the  body,  but 
without  combining ;  one  is  not  wholly  expelled,  as  in  the  case  of  the  electric 
fluid.  The  opposite  fluids  nullify  each  other  at  the  centre  of  the  magnetized 
body,  but  not  at  the  extremities,  which  become  their  chief  seats  of  action. 
If  a  new  extremity  is  formed  by  breaking  a  magnet,  a  new  pole  is  formed, 
opposite  in  kind  to  the  one  at  the  other  end.  "When  a  piece  of  iron  or  steel  is 
brought  near  the  positive  pole  of  a  magnet,  its  neutral  magnetic  fluid  is  de- 
composed. The  negative  portion  is  attracted  by  the  positive  pole  towards 
the  end  nearest  it,  which  consequently  becomes  a  negative  pole ;  while  the 
positive  element  is  repelled  towards  the  other  end,  and  forms  there  a  posi- 
tive pole. 

883.  TEKRESTRIAL  MAGNETISM. — The  polarity  of  the 
needle  is  best  explained  by  supposing  the  earth  itself  to  be 
a  vast  magnet.  At  the  magnetic  equator,  as  at  the  centre 
of  a  bar  magnet,  the  two  fluids  neutralize  each  other,  and 
there  are  no  magnetic  phenomena.  Hence  at  this  line  there 
is  no  dip.  The  chief  seats  of  magnetic  energy  are  two 
points  which  lie  towards  the  geographical  poles  of  the 
earth,  and  which  are  called  its  Magnetic  Poles. 

That  point  of  the  earth  which  attracts  the  north  or  positive  pole  of  the 
needle,  must  be  its  south  or  negative  magnetic  pole.  It  lies  near  Hudson's 

Ited  by  loadstone,  soft  iron,  and  steel,  explained?  How  is  it  with  most  substances  ? 
When  the  two  magnetic  fluids  are  separated,  where  do  they  seem  to  take  up  their 
abode  ?  Where  do  the  two  fluids  nullify  each  other,  and  where  not  ?  What  follows 
if  a  new  extremity  is  formed  by  breaking  a  magnet  ?  What  follows  when  a  piece  of 
steel  is  brought  near  the  positive  pole  of  a  magnet  ?  888.  How  is  the  polarity  of  tho 
needle  explained?  Why  is  there  no  dip  at  the  magnetic  equator?  What  is  meant 


TERRESTRIAL  MAGNETISM.  345 

Bay,  in  70  degrees  of  north  latitude,  and  was  reached  by  Captain  Ross  dur- 
ing his  Arctic  expedition  of  1829.  At  this  point  he  found  the  dipping  needle 
to  stand  vertical,  with  its  north  pole  towards  the  earth.  The  north  or  posi- 
tive magnetic  pole  of  the  earth  has  never  been  exactly  reached,  but  is  sup- 
posed to  lie  south  of  New  Holland,  in  about  75°  south  latitude.  The  dipping 
needle  would  there  also  stand  vertical,  but  with  its  south  pole  towards  the 
earth.  A  point  has  been  found  near  the  region  alluded  to,  in  which  the 
needle  is  very  nearly  vertical,  the  dip  being  88%  degrees. 

The  changes  in  the  variation  and  dip  appear  to  be  in  some  way  connected 
with  the  solar  heat  received  by  the  earth. 

884.  Magnets  draw  small  pieces  of  iron  to  themselves  ;  but  it  must  be 
remembered  that  the  magnetic  attraction  of  the  earth  only  affects  the  direc- 
tion, and  does  not  tend  to  change  the  actual  position.  A  magnetic  needle 
mounted  on  a  cork  and  placed  on  the  surface  of  a  pond,  is  made  to  point  north 
and  south  by  the  earth's  magnetic  attraction,  but  is  not  drawn  to  the  north 
side  of  the  pond. 

885.'  Magnetic  Intensity.  —  A  magnetic  needle  suspend- 
ed by  a  delicate  fibre,  when  turned  from  the  direction  in 
which  it  naturally  rests,  resumes  it,  but  not  immediately. 
The  magnetic  attraction  of  the  earth  brings  it  back,  but  its 
inertia  carries  it  past  the  point,  and  thus  a  series  of  vibra- 
tions, like  those  of  a  pendulum,  take  place  before  it  finally 
settles.  The  number  of  such  vibrations  occurring  in  a 
given  time  evidently  depends  on  the  intensity  of  the  earth's 
magnetic  attraction.  Now  this  number  (and  consequently 
the  intensity  of  terrestrial  magnetism)  is  found  to  be  dif- 
ferent at  different  places,  and  at  different  times  in  the  same 
place. 


Irations  made  in  a  given  time.  By  applying  this  law,  it  is  ascertained  that 
the  greatest  magnetic  intensity  thus  far  found  on  the  earth's  surface  is  three 
times  as  great  as  the  least.  The  magnetic  intensity  is  found  to  be  least  in 
Southern  Africa. 

Production  of  Artificial  Magnets. 

886.  Artificial  magnets  should  be  made  of  well  hard- 
ened steel,  of  fine  grain  and  uniform  structure,  free  from 

by  the  Magnetic  Poles  ?  Where  is  the  earth's  south  magnetic  pole  ?  By  whom  was 
it  reached,  and  what  was  found  there  ?  Where  is  the  earth's  north  magnetic  pole  f 
How  near  has  it  been  reached  ?  With  what  do  the  changes  in  variation  and  dip  seem 
to  be  connected  ?  884.  What  alone  is  affected  by  the  magnetic  attraction  of  the 
«arth  ?  Give  an  illustration.  885.  How  is  the  intensity  of  the  earth's  magnetic  at- 
traction shown  to  be  different  at  different  places  ?  What  is  the  law  for  ascertaining 

15* 


346  MAGNETISM. 

flaws,  and  having  level  and  polished  faces.  The  breadth 
of  a  bar  magnet  should  be  one-twentieth  of  its  length,  and 
its  thickness  about  one-seventieth  of  its  length.  In  a  horse- 
shoe magnet,  the  distance  between  the  poles  ought  not  to 
be  greater  than  the  breadth  of  one  of  the  sides. 

887.  Magnetism  may  be  imparted  to  steel  or  iron  in  four 
different  ways: — 1.  By  induction.     2.  By  the  sun's  rays. 
3.  By  contact  with  a  magnet.     4.  By  electric  currents. 

888.  INDUCTION,  A  SOURCE  OF  MAGNETISM. — A  magnetic 
atmosphere  surrounds  every  magnet.     A  piece  of  iron  or 
steel  brought  within  this  atmosphere,  even  without  touch- 
ing the  magnet,  has  its  neutral  fluid  decomposed,  and  ex- 
hibits magnetic  properties.   It  is  then  said  to  be  magnetized 
by  induction. 

Present  half  a  dozen  bars  of  iron  at  different  angles  to  the  positive  pole 
of  a  magnet,  without  letting  them  touch  it.  They  will  all  be  magnetized  by 
Induction,  the  ends  towards  the  magnet  becoming  negative  poles  and  the 
opposite  ends  positive. 

Suspend  two  pieces  of  soft  iron  wire  by  threads,  parallel  to  each  other 
nnd  on  the  same  level.  On  bringing  either  pole  of  a  magnet  a  short  distance 
below  them,  they  become  magnetized  by  induction.  Like  poles  are  formed 
in  their  contiguous  extremities,  and  consequently  instead  of  hanging  parallel 
as  before,  they  repel  each  other  and  diverge. 

Bring  one  end  of  an  unmagnetized  steel  bar  near  the  north  pole  of  a  mag- 
netic needle,  and  the  latter  will  be  attracted  to  it.  Now  place  the  positive 
pole  of  a  powerful  magnet  near  the  other  end  of  the  bar,  and  the  needle  will 
soon  be  repelled.  This  is  because  the  bar  becomes  magnetized  by  induction. 
The  end  nearest  the  needle  becomes  a  positive  pole  by  which  the  positive 
pole  of  the  latter  is  repelled. 

889.  The  earth  magnetizes  by  induction.     A  bar  of  soft 
iron  placed  in  the  direction  of  the  dipping  needle,  acquires 
magnetic  properties  by  the  inductive  influence  of  the  earth 
acting  as  a  magnet.     A  few  blows  with  a  hammer  on  the 

the  magnetic  intensity  ?  What  is  found  by  applying  this  law  ?  Where  is  the  mag- 
netic intensity  found  to  be  least  ?  886.  Of  what  should  artificial  magnets  be  made  ? 
What  should  be  the  comparative  dimensions  of  a  bar  magnet  ?  What  is  essential  in 
a  horse-shoe  magnet?  8ST.  Name  the  four  ways  in  which  magnetism  may  be  im- 
parted to  a  piece  of  steel  or  iron.  888.  When  is  a  piece  of  iron  said  to  be  magnetized 
~by  induction?  Illustrate  magnetic  induction  with  an  experiment.  Describe  the 
experiment  with  two  pieces  of  soft  iron  wire.  What  other  experiment  proves  that  a 
bar  may  be  magnetized  by  induction  ?  8S9.  How  is  it  proved  that  the  earth  mag- 
netizes by  induction  *  What  experiment  shows  the  inductive  influence  of  the  earth  ? 


PRODUCTION   OF   ARTIFICIAL  MAGNETS.  34*7 

upper  end,  by  causing  the  particles  to  vibrate,  help  them 
to  receive  the  magnetic  influence. 

Hold  a  bar  of  soft  iron  horizontally  with  one  end  near  the  north  pole  of  a 
magnetic  needle.  The  iron,  being  unmagnetized,  attracts  the  needle.  Now 
hold  the  bar  in  the  direction  of  the  dipping  needle,  give  it  one  or  two  blows 
with  a  hammer,  and  the  north  pole  of  the  needle  will  be  repelled, — showing 
that  the  bar  is  magnetized,  and  a  north  pole  formed  in  its  lower  end,  by  the 
inductive  influence  of  the  earth. 

Iron  bars  that  have  long  stood  in  a  vertical  position,  or  in  the  direction 
of  the  dipping  needle,  often  acquire  magnetic  properties  in  an  inferior  de- 
gree. The  same  may  be  said  of  iron  bars  raised  to  a  red  heat  and  allowed 
to  cool  in  the  positions  above  mentioned,  as  well  as  of  augers,  gimlets,  &c., 
that  have  been  much  used.  Iron  wire  is  frequently  made  magnetic  by  twist- 
ing it  till  it  breaks. — All  these  are  instances  of  magnetism  by  induction. 

890.  THE  SUN'S  RAYS,  A  SOURCE  OF  MAGNETISM. — Sun- 
light constitutes  a  second  source  of  magnetism.    The  violet 
rays  of  the  solar  spectrum,  concentrated  by  lenses  on  steel 
needles,  have  been  found  to  endow  them  with  magnetic 
properties. 

891.  CONTACT  WITH  A  MAGNET,  A  SOURCE  OF  MAGNET- 
ISM.— A  third  and  more  efficient  mode  of  exciting  magnet- 
ism in  iron  or  steel  is  by  bringing  it  in  contact  with  a  mag- 
net.    Till  recently  this  was  the  way  in  which   artificial 
magnets  were  almost  exclusively  produced. 

There  are  several  different  ways  of  magnetizing  by  con- 
tact.    The  principal  are  as  follows  : — 

892.  Magnetizing  Needles. — An  ordinary  sewing  needle 
may  be  magnetized  by  simply  touching  one  of  its  ends  to 
either  pole  of  a  powerful  magnet.     The  end  in  question  be- 
comes negative  if  touched  to  the  positive  pole,  and  positive 
if  touched  to  the  negative. 

893.  Magnetizing  Bars. — Steel  bars  maybe  magnetized 
either  by  single  touch  or  double  touch.     Single  Touch  con- 
sists in  applying  but  one  pole  of  a  magnet  to  the  bar,  or 
one  pole  to  one-half,  and  the  opposite  pole  to  the  other. 


Give  some  further  instances  of  magnetism  by  tho  inductive  influence  of  the  earth. 
890.  What  is  a  second  source  of  magnetism  ?  How  may  sun-light  be  made  to  mag«- 
ftetize  steel  needles  ?  891.  What  is  a  third  source  of  magnetism  ?  892.  What  is  the 
mode  of  magnetizing  needles  ?  893.  "What  two  modes  are  there  of  magnetizing  steel 


348  MAGNETISM. 

Double  Touch  consists  in  applying  both  poles  at  the  same 
time  throughout  the  whole  length  of  the  bar. 

894.  To  magnetize  a  bar  by  single  touch,  apply  midway  of  its  length  one 
of  the  poles  of  a  magnet,  and  draw  it  to  either  end.     Return  it  through  the 
air  to  the  middle  of  the  bar,  and  draw  it  again  to  the  same  end  as  before. 
Repeat  this  process  several  times,  always  using  the  same  pole  and  drawing 
it  in  the  same  direction.     Then  place  the  other  pole  on  the  middle  of  the  bar, 
and  draw  it  to  the  opposite  extremity,  repeating  the  strokes  as  in  the  former 
case.     This  must  be  done  on  both  sides  of  the  bar. 

Another  mode  is  repre- 
sented in  Fig.  311.  The  op- 
posite poles  of  two  magnets, 
kept  about  one-fourth  of  an 
inch  apart  by  apiece  of  wood, 
are  placed  on  the,  centre  of 
the  bar  A  B,  so  as  to  form  angles  of  about  30  degrees  with  its  surface.  They 
are  then  slowly  drawn  in  contrary  directions  from  the  middle  to  the  extrem- 
ities. This  process  is  repeated  several  times,  the  magnets  being  raised  when 
they  reach  the  ends  and  replaced  in  the  middle.  The  bar  is  then  turned  over, 
and  the  same  thing  done  on  the  other  side.  The  process  is  facilitated  by 
resting  the  ends  of  the  bar  on  the  opposite  poles  of  two  other  magnets,  as 
$hown  in  the  figure. 

895.  To  magnetize  a  bar  by  double  touch,  apply  the  opposite  poles  of  two 
magnets  as  just  described,  only  let  them  be  perpendicular  to  the  surface. 
Then,  instead  of  drawing  them  to  opposite  extremities  as  before,  move  them 
together  from  the  middle  to  one  end,  then  through  the  air  to  the  opposite  ex- 
tremity, and  over  the  bar  to  the  same  end  again,  and  so  on — drawing  them 
in  the  same  direction  over  the  bar,  letting  neither  of  the  applied  poles  pass 
beyond  its  extremity,  and  finally  stopping  in  the  middle. 

Fig  312>  896.  Magnetizing  Horse-shoe  Bars. 

— Horse-shoe  magnets  are  produced  by 
placing  a  piece  of  soft  iron,  as  a  keeper, 
across  the  ends  of  a  steel  bar  bent  in  the 
proper  form  ;  and  then,  as  shown  in  Fig. 
312,  applying  perpendicularly  to  the  ex- 
tremities a  horse-shoe  magnet,  whose 
arms  are  the  same  distance  apart.  Move 
it  slowly  to  the  bend,  then  carry  it  back  through  the  air  to 
the  extremities,  and  draw  it  to  the  bend  again.  This  must 

bars  ?  In  what  does  Single  Touch  consist  ?  In  what,  Double  Touch  ?  894.  Describe 
the  process  of  magnetizing  a  bar  by  single  touch.  What  other  mode  is  described  ? 
39Jx  How  is  a  bar  magnetized  by  double  touch  ?  896.  How  are  horse-shoe  magnets 


L    '         ^         *4 

PRODUCTION   OF  ARTIFICIAL  MAGNETS.   C    •      349  ' 

^  v/         *\y 

be  done  about  a  dozen  times ;  then,  withouft^mSmDgii^ ^ 
keeper,  turn  the  bar  over  and  do  the  same  on  the  otfo^pide.^ 
The  poles  of  the  magnet  produced  will  in  this  case  i>e!<5f 
the  same  character  as  those  respectively  brought  in 
tact  with  them. 

897.  The  best  mode  of  magnetizing  a 
horse-shoe  bar  is  represented  in  Fig.  313. 
Lay  the  horse-shoe,  A  B,  flat  on  a  table, 
with  its  ends  in  contact  with  the  poles  of 
a  horse-shoe  magnet,  N,  S.  Then  place 
a  piece  of  soft  iron  on  these  poles,  and 
draw  it  slowly  six  or  eight  times  towards 

the  bend  of  the  bar,  in  the  direction  of  the  arrow,  raising  it  as  often  as  it 
reaches  the  bend,  and  replacing  it  as  at  first.  This  process  performed  on 
both  sides  endows  the  horse-shoe  with  strong  magnetic  properties.  The  end 
which  touches  the  positive  pole  of  the  horse-shoe  magnet  becomes  negative, 
and  the  other  positive. 

Two  straight  bars  may  be  readily  magnetized  at  once  in  the  same  way, 
by  placing  one  extremity  of  each  against  the  poles  of  the  horse-shoe  magnet, 
and  connecting  the  opposite  ends  with  a  keeper. 

898.  ELECTRIC  CURRENTS,  A  SOURCE  OF  MAGNETISM. — 
A  bar  of  iron  or  steel  is  endowed  with  magnetic  properties 
in  the  highest  degree,  by  passing  a  current  of  voltaic  elec' 
tricity  over  a  conductor  placed  in  a  certain  position  rela* 
tively  to  the  bar.     The  details  of  this  process  belong  to 
that  branch  of  the  science  which  is  known   as  Electro- 
magnetism. 

Electro-magnetism. 

899.  Electro-magnetism  treats  of  the  phenomena  and 
principles  of  magnetism  excited  by  the  passage  of  electric 
currents. 

900.  EFFECTS  OF  ELECTRIC  CURRENTS  ON  THE  MAGNETIC 
NEEDLE. — As  a  science,  Electro-magnetism  owes  its  origin 
to  a  discovery  made  in  1819  by  Prof.  Oersted,  of  Copen- 
hagen.    He  found  that  a  wire  along  which  a  voltaic  current 

produced?  What  will  be  the  character  of  the  poles  in  the  magnet  produced? 
897.  With  Fig.  313,  describe  the  best  mode  of  magnetizing  a  horse-shoe  bar.  How 
may  two  straight  bars  be  magnetized  at  once  ?  898.  How  is  a  bar  of  steel  endowed 
with  magnetic  properties  in  the  highest  degree  ?  899.  Of  what  does  Electro-magnet- 
ism treat  ?  900.  To  what  does  electro-magnetism  owe  its  origin  ?  Give  an  account 


350  ELECTRO-MAGNETISM. 

was  passing  tended  to  turn  the  magnetic  needle  from  its 
natural  position  to  one  perpendicular  to  the  direction  of 
the  current.  The  conducting  wire,  of  whatever  metal  it 
might  be,  was  thus  rendered  magnetic  by  the  electric  cur- 
rent which  it  transmitted.  It  was  subsequently  found  to 
attract  iron  filings ;  which,  when  the  battery  was  in  full 
action,  clustered  around  it  to  the  thickness  of  a  quill,  but 
gradually  thinned  off  as  the  energy  of  the  battery  dimin- 
ished, and  left  it  entirely  bare  the  moment  the  circuit  was 
broken. 

The  direction  in  which  the  needle  is  turned  depends  on  its  position  rela- 
tively to  the  wire,  and  the  direction  in  which  the  current  is  passing.  When 
the  needle  is  on  a  different  level  from  the  wire,  that  is,  directly  above  or  be- 
low it,  it  retains  its  horizontal  position ;  but  its  north  pole  is  turned  east  or 
west,  according  to  whether  it  is  above  or  below  the  wire,  and  according  to 
the  direction  in  which  the  current  moves.  When  the  needle  is  on  the  same 
level  with  the  wire,  but  on  one  side  of  it,  it  does  not  then  swerve  east  or 
Vvest ;  but  its  north  pole  is  made  either  to  dip  or  to  rise,  according  to  the 
side  of  the  wire  it  is  on  and  the  direction  in  which  the  current  moves.  The 
following  rule  enables  us  always  to  determine  the  direction  in  which  the 
needle  will  be  turned : — 

Imagine  yourself,  with  arms  extended  perpendicularly,  lying  along  the 
conducting  wire,  with  your  head  towards  the  point  from  which  the  current  is 
coming,  and  your  face  turned  towards  the  north  pole  of  the  needle  ;  then  this 
north  pole  will  be  deflected  in  the  direction  of  your  right  hand,  w?t,ether  it  l« 
up  or  down,  east  or  west. 

The  magnetic  influence  of  the  electric  current  is  not  therefore  exerted  in 
the  plane  of  the  conducting  wire,  but  rather  perpendicularly  to  that  plane, 
so  as  to  produce  circular  motion  round  the  wire. 

901.  The  deflection  of  the  needle  by  an  electric  cur- 
rent may  be  shown  with  the  apparatus  represented  in 
Fig.  314. 

A  brass  wire  is  bent  into  rectangular  form,  and  provided  with  a  screw- 
cup  at  each  extremity,  P,  N,  for  the  reception  of  the  wires  from  a  galvanic 
battery,  so  that  a  current  may  be  passed  above  and  below  \  magnetic  needle, 
N,  S,  suspended  within  the  rectangle.  The  arms  proceeding  from  P  and  N 


of  Oersted's  discovery.  How  was  it  proved  that  the  conducting  wire  was  rendered 
magnetic  by  the  electric  current?  On  what  does  the  direction  in  which  the  needlo 
turns  depend  ?  How  does  it  turn,  when  on  a  different  level  from  the  wire  ?  How, 
when  on  the  same  level  with  the  wire,  but  on  one  side  of  it  ?  State  the  rule  for  de- 
termining the  direction  in  which  the  needle  will  be  turned  ?  How  is  the  magnetic 
Influence  of  the  electric  current  exerted  ?  901.  Illustrate  the  deflection  of  the  needle 


THE   GALVANOMETER. 


351 


a? 


are  insulated  from  each  other  Fig.  314. 

where  they  cross.  No  sooner  is  a 

positive  current  passed  over  the 

upper  wire  from  north  to  south, 

than  the  needle  is  turned,  its 

north  pole  deviating  towards  the 

east  and  its  south  pole  to  the 

west. 

Here  the  under  current,  pass- 
ing in  the  opposite  direction  to 
the  upper  one,  tends  to  turn  the 
needle  in  the  same  direction ;  and  the  deflecting  force,  as  it  is  called,  is  there- 
fore twice  as  great  as  if  the  current  passed  in  one  direction  only.  If  the  wire 
be  bent  so  as  to  make  two  rectangles  about  the  needle,  the  deflecting  force 


Fig.  315. 


will  be  twice  as  great  as  when  but  one  is 
formed ;  if  five  rectangles  are  made,  as  in 
Fig.  315,  it  will  be  five  times  as  great,  &c. 
In  these  cases,  the  wire  must  be  covered 
with  silk  thread,  or  some  other  non-con- 
ductor, so  as  to  insulate  its  arms  from 
each  other,  and  oblige  the  current  to  traverse  its  whole  length.  It  is  on  this 
principle  that  the  Galvanometer  is  constructed. 

902.  Tlie  Galvanometer. — The  Galvanometer  is  an  in- 
strument for  measuring  the  force  of  galvanic  currents  by  the 
deflection  of  the  magnetic  needle.  It  consists  of  a  long 
wire  bent  into  an  oval  or  rectangular  coil,  the  parts  of 
which  are  prevented  from  touching  by  being  wound  with 
silk.  The  wire  terminates  in  screw-cups,  for  convenience 
of  connection  with  a  galvanic  battery.  Within  the  coil  a 
magnetic  needle  is  delicately  poised ;  and  the  instrument 
is  placed  so  that  the  wire  may  have  the  same  direction  as 
the  needle.  They  retain  this  direction  till  a  galvanic  cur- 
rent passes  over  the  wire,  when  the  needle  is  turned  to- 
wards the  east — more  or  less,  according  to  the  force  of  the 
current.  A  graduated  scale  fixed  below  the  needle,  with 
its  circumference  divided  into  degrees,  measures  the  de- 
flection, and  consequently  the  quantity  of  electricity  passing 
over  the  wire. 


with  Fig.  314.  What  is  the  effect  of  having  two  currents,  one  above  and  one  below  ? 
What  is  the  effect  of  having  two  rectangles  ?  Five  rectangles?  In  these  cases,  what 
nrecaution  must  be  taken?  What  instrument  is  constructed  on  this  principle? 
002.  What  is  the  Galvanometer  ?  Describe  the  galvanometer.  003.  How  is  the  gal- 


352 


ELECTRO-MAGNETISM. 


GALVANOMETER  WITH    ASTATIC 
NEEDLE. 


903.  Galvanometer  with  Astatit 
Needle. — Instead  of  the  ordinary  nee- 
dle, an  astatic  needle  (see  §  875)  is 
sometimes  used  in  the  galvanometer. 
In  this  case,  the  needle,  having  its 
polarity  neutralized,  is  more  readily 
turned.  The  instrument  is  consequent- 
ly more  sensitive,  indicating  the  pres- 
ence of  electric  currents  which  would 
otherwise  entirely  escape  detection. 

Fig.  316  represents  the  Galvanom- 
eter with  the  Astatic  Needle.  The  nee- 
dles are  suspended  by  two  parallel  silk 
threads  from  r,  so  that  one  of  them 
may  hang  directly  over  the  top  of  the 
coil  z  c,  and  the  other  below  it.  p  q  are 
the  screw-cups  terminating  the  wire 
which  forms  the  coil,  and  ss  is  the 
graduated  scale.  The  upper  needle 
hangs  above  the  coil ;  but  as  its  poles 
point  in  opposite  directions  to  those  of  the  under  one,  it  will  tend  to  move  in 
the  same  direction  as  the  latter  when  galvanic  action  takes  place. 

904.  CONNECTION  BETWEEN  ELECTRICITY  AND  MAGNET- 
ISM.— That  there  is  an  intimate  connection  between  elec- 
tricity and  magnetism,  was  established  by  Oersted's  experi- 
ment. It  is  further  shown  by  the  fact  that  compass-needles 
often  have  their  poles  reversed  or  their  polarity  weakened 
by  lightning ;  that  a  spark  has  been  drawn  from  a  magnet ; 
that  a  charge  of  electricity  passed  through  a  needle  renders 
it  magnetic ;  and  that  a  bar  may  be  permanently  magnet- 
ized with  an  electric  current  more  efficiently  than  in  any 
other  way. 

These  facts  have  led  to  the  theory  that  magnetism  is 
not  an  independent  agent,  but  simply  one  of  the  forms  as- 
sumed under  certain  circumstances  by  that  subtile  all- 
pervading  agent  which  we  call  THE  ELECTRIC  FLUID.  Ac- 
cording to  this  theory,  frictional  electricity,  voltaic  elec- 
tricity, thermo-electricity,  magneto-electricity,  and  electro- 
magnetism,  are  all  one  and  the  same  thing,  identical  in 

vanometcr  made  more  sensitive,  and  why  ?  Describe  the  Galvanometer  with  the 
Astatic  Needle.  904.  What  was  established  by  Oersted's  experiment?  How  is  the 
tonncction  between  electricity  and  magnetism  further  shown  ?  What  theory  has 


ELECTRO-MAGNETIC  KOTATION.  353 

kind,  but  differing  in  intensity,  quantity,  and  properties, 
in  consequence  of  the  different  modes  in  which  they  are 
developed. 

905.  ELECTKO-MAGNETIC  ROTATION. — When  a  magnetic 
pole  and  a  wire  over  which  an  electric  current  is  passing 
are  brought  near  each  other,  the  pole  tends  to  revolve 
round  the  wire,  and  the  wire  has  a  similar  tendency  to 
revolve  round  the  magnet  in  a  plane  perpendicular  to 
the  direction  of  the  current.  With  suitable  apparatus,  the 
following  phenomena  of  electro-magnetic  rotation  may  be 
exhibited : — 

1.  The  conducting  wire  being  fixed,  the  magnet  will 
revolve  about  it. 

2.  The  magnet  being  fixed,  the  conducting  wire  will 
revolve  about  it. 

3.  Both  magnet   and  wire  being  left  free  to   move, 
tney  will    revolve   in  the  same   direction    round   a  com- 
mon centre,  each  appearing  to  pursue  and  be  pursued  by 
the  other. 

4.  The  conducting  wire  being  dispensed  with,  a  magnet 
may  be  made  to  turn  on  its  own  axis  by  the  passage  of  an 
electric  current  along  half  its  length. 

906.  To  show  the  revolution  of  a  magnet  about  a 
conducting  wire,  Faraday  used  the  apparatus  repre- 
sented in  Fig.  317.  A  magnet,  n  S,  is  immersed  in  a 
vessel  of  mercury,  with  its  north  pole,  n,  a  short  dis- 
tance above  the  liquid,  and  its  south  pole,  S,  connect- 
ed by  a  silk  thread  with  the  conducting  wire  C,  which 
passes  through  the  bottom  of  the  vessel,  a  b  is  an- 
other conducting  wire,  which  enters  the  mercury  from 
above.  When  a  b  is  connected  with  the  positive  pole 
of  a  galvanic  battery,  and  C  d  with  the  negative,  a  de- 
scending current  of  positive  electricity  passes  along 
the  conductor  (the  mercury  completing  the  circuit), 
and  the  north  pole,  n,  will  revolve  round  the  fixed 
wire,  a  b,  in  the  direction  of  the  hands  of  a  watch.  If, 
on  the  contrary,  a  b  be  connected  with  the  negative 

been  based  on  these  facts  ?  905.  What  follows  when  a  magnetic  pole  and  a  wire  over 
which  an  electric  current  is  passing  are  brought  near  each  other?  With  suitable  ap- 
paratus, what  phenomena  connected  with  electro-magnetic  rotation  may  be  exhibit- 
ed? 906.  Describe  Faraday's  experiment  for  showing  the  revolution  of  a  magnet 


354 


ELECTRO-MAGNETISM. 


pole,  and  C  d  with  the  positive,  an  ascending  current  will  be  formed,  and  the 
magnet  will  revolve  in  the  opposite  direction. 

Mercury  is  used  in  this  experiment,  because,  being  a  liquid,  it  allows  the 
magnet  to  move  through  it,  while  at  the  same  time,  being  a  conductor,  it 


Fig.  818. 


Fig.  319. 


completes  the  circuit,  and  carries  off  the  magnetic  in- 
fluence from  the  south  pole  immersed  in  it.  Were  it 
not  for  this,  the  south  pole,  by  its  tendency  to  move 
in  the  opposite  direction  to  the  north,  would  keep  the 
magnet  stationary. 

907.  Fig.  318  illustrates  the  revolution  of  a  con- 
ducting wire  around  a  fixed  magnet.  Again  we  have 
a  vessel  of  mercury,  with  a  conducting  wire,  d,  passing 
through  its  bottom,  and  another  wire,  a  J,  suspended 
from  a  hook  directly  over  the  magnet,  entering  the 
mercury  from  above,  n  is  the  north  pole  of  the  fixed 
magnet.  On  connecting  the  hook  and  the  wire  d  with 
the  poles  of  a  galvanic  battery,  the  wire  will  revolve 
round  the  magnet,  the  direction  depending,  as  before,  on  whether  the  electric 
current  is  ascending  or  descending. 

908.  By  ingeniously  combining 
the  two  pieces  of  apparatus  just  de- 
scribed, we  may  exhibit  the  simulta- 
neous revolution  of  both  magnet  and 
wire  round  a  common  centre.  The 
magnet,  M,  is  immersed  in  a  vessel 
of  mercury  about  half  its  length,  that 
the  current  may  affect  only  one  pole. 
It  is  connected  at  the  bottom  with  a 
conducting  wire  and  screw-cup,  C,  in 
such  a  way  as  to  allow  it  freedom  of 
revolution.  The  wire,  "W,  is  sus- 
pended from  a  hook,  so  as  to  move 
freely.  On  transmitting  a  current, 
which  is  done  by  connecting  A  and 
C  with  the  poles  of  a  battery,  both 
the  magnet  and  the  wire  commence 
revolving  in  the  same  direction  as  if 
chasing  one  another 

909.  EFFECT  OF  ELECTRIC 
CURRENTS  ON  STEEL  AND  SOFT 
IRON. — The  deflection  of  a 
magnetic  needle  by  a  wire 

about  a  conducting  wire.  Why  is  mercury  used  in  this  experiment  ?  907.  Describe 
tne  experiment  which  shows  the  revolution  of  a  conducting  wire  around  a  fixed 
magnet.  908.  What  does  Fig.  819  represent  ?  Describe  the  experiment  with  this 


THE   HELIX. 

over  which  an  electric  current  is  passing,  ha^vhfeQn  cl^y 
scribed  in  §  900.     If  a  bar  of  soft  iron  is  placed  acrdss/such.    v 
a  wire,  it  becomes  a  temporary  magnet,  as  is  shown  r3y<4jttk 
attracting  iron  filings.     A  bar  of  steel  so  placed  is  made  &r  > 
permanent  magnet. 

910.  The  Helix. — The  magnetizing  power  of  the  wire  is 
greatly  increased,  if,  instead  of  touching  the  bar  in  but  a 
single    point    where    they  Fig.  820. 

cross,  it  is  wound  a  number     ^,,^^^^=^^^,^^^0 

of  times  spirally  round  the 

latter,  as  shown  in  Fig.  320. 

Such  a  coil  of  wire  is  called  a  Helix  (plural,  hel'-i-ces). 

A  helix  may  be  familiarly  made  by  winding  some  copper  wire  tightly 
round  a  small  bottle,  and  then  drawing  the  bottle  out.  As  the  magnetizing 
power  of  the  helix  increases  with  the  number  of  times  that  the  electric  cur- 
rent passes  round  the  bar,  each  turn  of  the  wire  is  pushed  close  up  to  the 
one  before  it ;  and,  to  increase  the  effect  still  further,  several  coils  or  layers 
of  wire  may  be  formed,  one  on  top  of  another.  Direct  communication  be- 
tween contiguous  parts  of  the  wire  must  be  prevented  by  winding  silk  or 
some  other  insu-  Fig.  321. 

lating  material 
round  it.  When 
the  ends  of  the 
wire  are  connect- 
ed with  the  poles 
of  a  galvanic  bat- 
tery, the  current 
is  thus  obliged  to 
pass  through  its 
whole  length.  Fig. 

321  represents   a  A  HELES' 

helix  mounted  on  a  stand.  An  iron  bar  extending  through  the  centre  is  seen 
projecting  at  each  end. 

911.  Magnetizing  Power  of  the  Helix. — A  steel  bar 
introduced  within  a  helix  becomes  permanently  magnetized 
the  moment  an  electric  current  is  passed  over  the  wire.    A 
needle  laid  inside  of  it  is  sometimes  so  powerfully  acted  on 

apparatus.  909.  What  is  the  effect  of  a  wire  over  which  a  current  is  passing  on  a  bar 
of  soft  iron  placed  across  it  ?  On  a  bar  of  steel  so  placed  ?  910.  How  is  the  effect 
greatly  increased  ?  What  is  such  a  coil  of  wire  called  ?  How  may  a  helix  be  made  ? 
How  is  the  effect  of  the  helix  increased  ?  With  what  is  the  wire  covered,  and  why  ? 
What  does  Fig.  321  represent?  911.  What  is  the  effect  of  a  helix  on  a  steel  bar  i»- 


356 


ELECTEO-MAGNETISM. 


as  to  be  lifted  up  and  held  suspended  in  the  air  in  the  mid- 
dle of  the  helix.     A  bar  of  soft  iron  placed  in  the  same  po> 
sition  is  endowed  with  strong  magnetic  properties  for  the 
Fig.  322.  time,  but  instantly  loses  them  when  re- 

moved, or  when  the  current  ceases  to 
pass.  To  be  magnetized,  the  bar  must 
always  be  placed  lengthwise  of  the  helix, 
— that  is,  at  right  angles  to  the  direction 
in  which  the  current  is  passing. 

One  of  the  most  remarkable  effects  of  the  helix  is 
the  suspension  in  the  air,  without  any  visible  support, 
of  a  heavy  iron  bar  loaded  with  weights.  A  helix 
consisting  of  a  very  long  wire,  forming  several  coils 
one  upon  another,  and  charged  by  a  powerful  battery, 
is  held  in  a  vertical  position,  as  shown  in  Fig.  322. 
An  iron  bar  brought  within  the  helix  just  at  its  base, 
will  be  lifted  up  half  way  into  it,  and  held  there  in 
the  centre  of  the  hollow  cylinder,  without  touching 
it,  as  long  as  the  current  continues  to  pass.  If  pulled 
down  a  little  way,  it  immediately  springs  back  to  its 
former  position.  The  moment  the  current  ceases,  the 
bar  falls.  With  a  powerful  apparatus,  a  weight  of 
eighty  pounds  has  been  thus  kept  suspended  in  the 


air. 


Fig.  323. 


A  no  less  interesting  experiment, 
showing  the  power  of  the  helix,  may  be 
performed  with  the  apparatus  repre- 
sented in  Fig.  323.  The  helix,  A,  is  in 
the  form  of  a  ring.  B,  C,  are  two  semi- 
circular pieces  of  soft  iron,  having  their 
ends  accurately  fitted  to  each  other. 
When  B  and  C  are  brought  together  so 
as  to  form  a  circle,  with  one  pair  of  their 
joined  ends  within  the  helix,  they  are 
endowed  with  so  strong  an  attraction 
for  each  other  that  two  men  can  hardly 
pull  them  apart.  . 

912.  Electro-Magnets. — An 
electro-magnet  consists  of  a  bar  of  soft  iron  within  a  helix. 


troduced  within  it  ?  On  a  needle  ?  On  a  bar  of  soft  iron  ?  To  be  magnetized,  how- 
must  the  bar  be  placed?  What  is  one  of  the  most  remarkable  effects  of  the  helix  ? 
Describe  the  experiment.  Describe  tho  experiment  with  the  apparatus  represented 


ELECTROMAGNETS. 


357 


It  is  strongly  magnetic  as  long 
as  a  current  passes  over  the  wire, 
but  loses  its  power  the  moment 
the  current  ceases. 

The  most  powerful  electro-magnet  is 
made  by  bending  a  bar  of  soft  iron  into  the 
form  of  a  horse-shoe,  as  shown  in  Fig.  324, 
and  winding  closely  round  it  a  large  quan- 
tity of  insulated  copper  wire  so  as  to  form 
&  helix  of  several  layers.  The  ends  of  the 
wire,  Z,  C,  are  connected  with  a  powerful 
battery.  A  soft  iron  keeper,  P  N,  connects 
the  poles,  having  a  hook  beneath,  to  which 
weights  may  be  attached.  So  strongly  is 
this  keeper  attracted  that  an  enormous 
force  is  required  to  separate  it.  An  elec- 
tro-magnet prepared  as  above  has  sup- 
ported over  4,000  pounds. 

913.  Electro-magnets  furnish  us  with  the  most  efficient 
means  of  magnetizing  an  ordinary  horse-shoe  bar.  The 
mode  of  using  them  for  this  purpose  is  shown  in  Fig.  325. 

Fig.  325. 


AN  ELECTRO-MAGNET. 


The  electro-magnet  is  applied  at  the  bend,  one  pole  on  each  arm,  and 
drawn  towards  the  extremities,  N,  S.  This  is  done  several  times  on  both 
sides,  when  the  bar  is  rendered  permanently  magnetic.  To  deprive  it  of  its 
magnetic  power,  reverse  the  process,  by  applying  the  poles  of  the  electro- 
magnet to  the  ends  N,  S,  and  drawing  them  towards  the  bend. 

914.  ELECTRO-MAGNETISM,  AS  A  MOTIVE  POWER. — We 
have  seen  that  an  electro-magnet  is  instantly  endowed  with 

In  Fig.  828.  912.  Of  what  does  an  electro-magnet  consist  ?  How  is  the  most  power, 
ful  electro-magnet  made  ?  How  great  a  weight  has  been  supported  with  such  an 
electro-magnet  ?  913.  What  is  the  most  efficient  means  of  magnetizing  a  horse-sho« 


858  ELECTRO-MAGNETISM. 

great  attractive  power  for  iron  on  being  connected  with  a 
galvanic  battery,  and  as  instantly  divested  of  it  when  the 
connection  is  severed.  It  may  thus  be  made  to  impart 
motion  to  an  iron  rod,  and  through  it  to  various  kinds  of 
machinery.  So  strong  at  one  time  was  the  impression  that 
the  enormous  attractive  power  of  the  electro-magnet  could 
be  advantageously  used  as  a  mechanical  agent,  that  the 
United  States  government  appropriated  $20,000,  and  Russia 
$120,000,  for  experiments  on  the  subject ;  and  various  ma- 
chines were  contrived  in  which  it  was  used  as  a  motive 
power.  In  none,  however,  thus  far  invented,  has  it  been 
found  to  approach  steam  in  efficiency  or  economy. 

A  boat  28  feet  long  with  a  dozen  persona  on  board  has  been  propelled 
against  the  current  at  the  rate  of  three  miles  an  hour  by  electro-magnetic 
action.  A  locomotive  engine  has  also  been  driven  from  ten  to  twelve  miles 
an  hour.  But  this  is  the  utmost  that  has  been  effected,  and  in  both  cases 
the  cost  of  keeping  the  galvanic  battery  in  operation  was  much  greater  than 
that  of  producing  an  equivalent  quantity  of  steam.  The  difficulty  appears  to 
be  twofold.  First,  the  attractive  power  of  the  magnet  rapidly  diminishes  as 
the  distance  from  it  increases.  Secondly,  electric  currents  opposite  in  direc- 
tion to  the  primary  one  are  excited  in  the  moving  machinery ;  which,  in- 
creasing in  power  with  its  velocity,  nullify  much  of  the  effect  of  the  magnet. 
Until  these  difficulties  are  removed,  electro-magnetism  can  not  be  advan- 
tageously used  as  a  mechanical  agent. 

915.  THE  ELECTKO-MAGNETTC  TELEGEAPH. — Although 
unavailable  as  a  motive  power,  electro-magnetism  has  been 
turned  to  practical  account  in  the  Telegraph,  one  of  the 
crowning  triumphs  of  human  ingenuity.     For  this  great 
invention  as  at  present  perfected,  which  enables  us,  almost 
with  the  rapidity  of  thought,  to  communicate  with  distant 
points,  over  miles  of  intervening  land  or  sea,  the  world  is 
chiefly  indebted  to  an  American — Samuel  F.  B.  Morse. 

916.  Morse's  Telegraph. — The  principles  on  which  Morse's 
Telegraph  operates  are  as  follows : — 


bar?  Describe  the  process.  914  On  what  principle  may  an  electro-magnet  be  made 
to  impart  motion  to  an  iron  rod  ?  For  what  were  appropriations  made  by  the  United 
States  government  and  Russia  ?  What  has  been  effected  with  machinery  moved  by 
electro-magnetism?  How  does  the  expense  compare  with  that  of  steam?  What 
difficulties  interfere  with  the  usefulness  of  electro-magnetism  as  a  motive  power  ? 
915.  In  what  has  electro-magnetism  been  turned  to  practical  account?  To  whom  i» 


THE  ELBCTBO-MAGNETIC  TELEGRAPH. 


359 


1.  An  electro-magnet  may  be  alternately  endowed  with 
and  deprived  of  the  property  of  attracting  iron  by  connect- 
ing and  disconnecting  it  with  a  galvanic  battery. 

2.  The  battery  may  be  miles  away  from  the  magnet.    If 
wires  connect  the  two,  the  electric  current  will  still  be  car- 
ried to  the  helix  and  produce  the  same  effects. 

3.  A  person  stationed  near  the  battery  may  complete 
and  break  the  circuit  at  pleasure.    As  he  does  so,  one  end 
of  a  lever  placed  near  the  poles  of  the  distant  magnet  will 
be  attracted  or  released.     When  it  is  attracted,  the  other 
end  of  the  lever,  which  is  furnished  with  a  point,  is  made 
to  indent  a  strip  of  paper  passed  in  front  of  it  by  machinery, 
with  dots  or  dashes,  according  to  the  time  that  the  opera- 
tor by  the  battery  keeps  the  circuit  complete.     If,  now, 
different  combinations  of  dots  and  dashes  are  agreed  upon 
to  represent  certain  letters,  it  is  evident  that  a  message  can 
be  communicated  from  the  one  point  to  the  other. 

Fig.  326  represents  Morse's  recording  apparatus. 

Fig.  826. 


the  world  chiefly  indebted  for  the  Telegraph?    916.  State  the  principles  on  which 
Morse's  Telegraph  operates    Describe  Morsel  recording  apparatus,  and  its  mode  of 


360 


ELECTBO-MAGNETISM. 


A  B  is  the  electro-magnet,  connected  with  the  distant  battery  by  the  wires 
L,  M,  which  are  raised  on  poles  and  insulated  by  glass  supports.  C  is  an 
armature  of  soft  iron  attached  to  one  end  of  the  lever  D  D,  so  as  to  rest  about 
one-eighth  of  an  inch  above  the  poles  of  the  magnet.  The  other  end  of  th« 
lever  carries  a  point  or  style,  I,  which  is  raised  as  C  is  depressed.  A  strip 
of  paper,  F,  F,  rolled  on  the  spool  E,  is  made  to  pass  in  front  of  the  style, 
between  the  two  cylinders  Gr,  H,  by  means  of  wheel-work  set  in  motion  by 
the  weight  J  when  the  current  passes.  K  is  a  spring,  to  pull  down  the  end 
of  the  lever  bearing  the  style  when  the  other  end  is  released  by  the  magnet. 
A  striking  apparatus  was  formerly  connected  with  the  machinery  in  such  a 
way  as  to  give  warning  to  the  attendant  with  the  first  motion  of  the  lever ; 
but  it  is  now  generally  dispensed  with,  as  the  clicking  sound  produced  by 
the  lever  is  found  to  be  sufficient  for  the  purpose. 

Instead  of  carrying  both  wires  over  poles  from  the  electro-magnet  to  the 
battery,  the  earth  is  now  generally  made  to  form  one-half  the  circuit.  This 
is  effected  by  carrying  down  the  wire  from  the  magnet,  and  connecting  it 
with  a  metallic  plate  buried  in  the  ground ;  a  similar  plate  must  be  buried 
where  the  battery  is  stationed,  and  a  wire  from  the  latter  connected  with 
it.  If  this  is  done,  but  one  wire  need  pass  over  the  poles  to  complete  the 
circuit. 

917.  The  apparatus  used  by  the  operator  where  the 
battery  is  stationed,  to  complete  and  break  the  circuit,  is 
called  the  Signal  Key.  It  is  represented  in  Fig.  327. 

By  pressing  on  the  knob, 
the  screws  in  which  the  wires 
are  fastened  are  connected, 
and  the  circuit  is  completed. 
On  removing  the  hand,  the 
knob  springs  up,  the  circuit  is 
broken,  and  the  current  ceases. 
If  the  knob  is  kept  pressed 
down,  the  paper  at  the  other 
end  is  indented  with  a  contin- 
uous line ;  but  by  tapping  on 
it  so  as  to  form  different  com- 
binations of  dots  and  dashes, 
which  stand  for  letters,  and 
are  understood  at  both  ends  of  the  line,  a  message  is  transmitted.  Accord- 
ing to  Morse's  system,  the  following  combinations  are  used  to  represent  the 
different  letters  and  figures  : — 


Fig.  327. 


THE  SIGNAL  KEY. 


operation.  What  was  formerly  connected  wi*h  the  machinery  ?  Why  is  it  now  dis- 
pensed with  ?  Instead  of  carrying  both  wires  over  poles  from  the  electro-magnet  to 
the  battery,  what  is  now  the  more  usual  arrangement  ?  How  is  the  earth  made  to 
form  half  of  the  circuit?  91T.  What  is  the  Signal  Key?  Describe  it,  and  its  moda 


MORSE'S  TELEGRAPH. 


361 


LETTERS. 


a  
1  

;  

k  

7 

•    _          - 

d  

e  - 
f  

m  
w  .       — 

o  - 

g  , 
h  
t  -  - 

P  

q  

r  •  -  - 

FIGURES. 
1 

2  -   -    

3 

4 

5  - 


To  prevent  confusion,  a  small  space  is  left  after  each  letter,  a  longer  one 
between  words,  and  a  still  longer  one  at  the  end  of  a  sentence.  The  opera- 
tors in  telegraph  offices  become  so  familiar  with  this  alphabet  that  they  un- 
derstand a  message  from  the  mere  clicks  of  the  lever,  without  looking  at 
the  paper  on  which  it  is  recorded. 

918.  An  electric  current  is  transmitted  by  a  wire  to  a 
great  distance,  but  not  with  undiminished  power.  When, 
therefore,  the  stations  are  very  far  apart,  the  electro- 
magnet is  charged  too  feebly  to  make  the  style  indent  the 
paper.  In  this  case,  the  wire  from  the  original  battery  is 
made  to  act  on  a  very  delicate  armature,  so  as  to  complete 
the  circuit  of  a  second  battery  placed  near  the  machine. 
This  Relay  Battery,  as  it  is  called,  acts  on  the  recording 
apparatus  as  described  above,  or  transmits  a  fresh  and  vig- 
orous current  to  another  relay  battery.  In  this  way  lines 
of  any  length  may  be  formed. 

As  relay  batteries  do  not  interrupt  the  circuit,  any  number  of  them  may 
be  placed  at  intervals  along  a  line.  Each  may  work  a  recording  apparatus 
of  its  own,  and  a  given  communication  may  thus  be  registered  simultane- 
ously at  a  multitude  of  different  stations. 

Relay  batteries  may  be  dispensed  with  by  increasing  the  number  of  plates 
employed  and  distributing  them  in  groups  along  the  line.  It  has  been  com- 
puted that  if  a  telegraph  wire  could  be  carried  round  the  earth,  1200  of 
Grove's  pint  cups,  distributed  in  equi-distant  groups  of  fifties,  would  supply 
the  galvanic  power  for  the  whole  distance. 

«f  operation.  How  are  the  different  letters  represented?  918.  What  difficulty  is 
there  when  the  current  is  transmitted  to  a  great  distance  ?  How  is  this  remedied  ? 
How  does  the  Belay  Battery  act?  How  may  a  given  message  be  registered  simulta- 
neously at  different  stations  ?  What  may  be  substituted  for  relay  batteries  ?  How 
toany  cups  would  supply  the  galvanic  power  for  a  telegraph  round  the  earth? 

16 


362  ELECTEO-MAGNETISM. 

919.  Houses  and  JBairi>s  Telegraph. — Morse's  appara- 
tus, having  been  first  introduced  and  being  very  simple 
and  not  likely  to  get  out  of  order,  is  more  used  than  any 
other,  both  in  this  country  and  in  Europe.  There  are  other 
ingenious  systems,  however,  which  are  employed  to  some 
extent.    Among  these  are  House's  Printing  Telegraph  and 
Bain's  Electro-chemical  Telegraph. 

House's  apparatus  is  one  of  the  most  wonderful  achievements  of  invent- 
ive art.  Making  use  of  the  electro-magnet  in  connection  with  ingenious  and 
somewhat  intricate  machinery,  it  enables  the  operator,  by  playing  on  twenty- 
eight  keys  like  those  of  a  piano  (representing  the  twenty-six  letters  and  two 
punctuation  points),  to  print  ordinary  letters  on  a  strip  of  paper  at  the  other 
end  of  the  line  at  the  rate  of  about  two  hundred  a  minute.  The  great  advan- 
tages of  House's  system  are  that  there  is  little  or  no  liability  to  mistake  in 
transmitting  a  message,  and  that  the  latter,  being  produced  in  Roman  cap- 
itals, need  not  be  transcribed,  but  may  be  sent  just  as  it  comes  from  the 
machine  to  the  person  for  whom  it  is  intended. 

In  Bain's  Electro-chemical  Telegraph  no  magnet  is  used.  The  point  of 
the  wire,  which  is  stationary,  constitutes  the  pen,  and  rests  lightly  on  a  me- 
tallic plate,  which  is  made  to  revolve  by  machinery.  On  this  plate  is  placed 
paper  which  has  been  previously  moistened  with  some  chemical  preparation 
decomposable  by  voltaic  electricity.  When  the  connection  is  made  by  the 
distant  operator,  the  current  passes  from  the  wire  to  the  plate  through  the 
paper,  and  in  passing  decomposes  the  chemical  compound  with  which  the 
paper  is  impregnated.  The  result  is  a  deep  blue  spot  on  the  paper,  which 
renders  the  dot  or  dash  visible,  just  as  the  indentation  does  according  to 
Morse's  system.  As  even  a  feeble  voltaic  current  has  the  power  of  decom- 
position, there  is  not  the  same  necessity  for  relay  batteries  on  Bain's  line  as 
on  either  of  the  others. 

920.  Submarine  Telegraphs. — Submarine  Telegraphs  are 
telegraphs  connecting  points  separated  by  water,  in  which 
the  wire  is  submerged.     The  first  successful  telegraph  of 
this  kind  was  laid  in  1851  across  the  English  Channel,  and 
connected  Dover  with  the  French  coast.     This  was  fol- 
lowed by  several  others;  and  in  1858,  after  several  unsuc- 
cessful attempts,  a  telegraph  cable  nearly  2,000  miles  in 
length  was  laid  across  the  Atlantic  Ocean,  between  Valen- 

919.  What  other  telegraph  systems  besides  Morse's  are  in  use  ?  What  is  said  of 
House's  apparatus  ?  What  are  its  great  advantages  ?  What  is  the  principle  involved 
in  Bain's  Electro-chemical  Telegraph  ?  What  advantage  is  there  connected  with  tliis 
system?  920.  What  are  Submarine  Telegraphs?  Where  and  when  was  the  first 
submarine  telegraph  laid  ?  In  1853  what  great  enterprise  was  carried  through}  DC- 


HISTORY    OF  THE  TELEGRAPH.  86. 7 

tia  Bay,  Ireland,  and  Trinity  Bay  on  the  coast  of  New- 
foundland. It  consisted  of  a  group  of  seven  copper  wires 
insulated  and  protected  by  a  casing  of  gutta-percha,  the 
whole  surrounded  by  strands  of  iron  wire,  and  sunk  to  the 
bottom,  of  the  ocean,  at  a  depth  nowhere  exceeding  2^  miles. 

Public  interest  was  strongly  excited  in  this  great  enterprise ;  but  it  has 
thus  far  been  doomed  to  disappointment.  After  transmitting  several  mes- 
sages, the  Atlantic  Telegraph,  for  some  unexplained  reason,  ceased  to  work, 
though  signals  have  from  time  to  time  been  received.  There  is  little  doubt, 
however,  that  the  work  is  feasible,  and  that  we  shall  soon  have  regular  tele- 
graphic communication  between  the  opposite  sides  of  the  Atlantic. 

921.  History  of  the  Telegraph.— The  fact  that  frictional 
electricity  could  be  conveyed  by  wires  to  a  great  distance 
was  known  more  than  a  hundred  years  ago.     Franklin,  in 
1748,  set  fire  to  alcohol  by  means  of  a  wire  from  an  elec- 
trical machine  carried  across  the  Schuylkill  River.     The 
first  attempt  to  transmit  a  communication  by  electricity, 
however,  was  made  in  1774  by  Le  Sage  [luh  sahzh],  a 
Frenchman,  at  Geneva. 

Le  Sage  used  twenty-four  wires  insulated  in  glass  tubes  buried  in  the 
earth,  each  of  which  represented  a  letter  of  the  French  alphabet.  The  wires 
were  connected  with  an  electrical  machine  in  the  order  necessary  to  spell  out 
the  words,  and  electroscopes  attached  to  them  at  the  other  end  indicated  this 
order  by  their  successive  divergence  to  an  attendant  stationed  there. 

922.  Volta's  discovery  in  1800  furnished  afar  more  effi- 
cient agent  for  telegraphic  communication  than  frictional 
electricity,  and  was  followed  in  a  few  years  by  a  plan  for  an 
electro-chemical  telegraph,  requiring  thirty-five  wires,  to 
represent  the  different  letters  and  figures,  and  to  act  by 
the  decomposition  of  water. 

The  great  discovery  of  electro-magnetism  in  1819  called 
forth  many  new  suggestions, — among  others,  the  use  of  the 
deflections  of  the  needle  as  signals  ;  but  none  of  the  plans 
proposed  were  practicable  on  a  large  scale.  A  more  per- 

scribe  the  Atlantic  cable.  What  is  said  of  the  working  of  the  Atlantic  telegraph? 
921.  What  fact  relating  to  frictional  electricity  was  known  more  than  a  hundred  years 
ago?  What  experiment  was  performed  by  Franklin  in  1748?  Who  made  the  first 
attempt  to  transmit  a  message  by  electricity  ?  Describe  the  plan  of  Le  Sage.  922.  By 
what  was  the  discovery  of  voltaic  electricity  followed  ?  What  suggestions  were  called 


364  ELECTRO-MAGNETISM. 

manent  galvanic  power  was  needed ;  and  this  was  not  sup- 
plied till  1836,  when  Daniell  brought  out  his  constant  bat- 
tery. The  appearance  of  this  battery  and  the  improved 
electro-magnets  prepared  by  Prof.  Henry,  was  followed  in 
1837  by  the  invention  of  apparatus  for  transmitting  and 
recording  communications,  by  Samuel  F.  B.  Morse,  who 
had  been  experimenting  on  the  subject  for  five  years.  Ap- 
plication was  at  once  made  to  the  Congress  of  the  United 
States  for  aid  to  construct  a  line  of  sufficient  length  to  test 
the  invention;  and  after  discouraging  delays,  in  1843,  the 
sum  of  $30,000  was  appropriated  by  that  body,  with  which 
a  line  was  established  between  Baltimore  and  Washington, 
a  distance  of  forty  miles.  The  enterprise  was  crowned  with 
complete  success ;  and  the  first  news  transmitted  was  the 
proceedings  of  the  democratic  convention  of  1844,  then 
sitting  in  Baltimore,  by  which  James  K.  Polk  was  nomi- 
nated for  the  presidency. 

So  manifold  were  the  advantages  of  telegraphic  communication,  that  im- 
mediately on  the  announcement  of  Morse's  success  companies  were  formed, 
and  wires  were  soon  seen  threading  the  country  in  all  directions.  The  va- 
rious lines  now  in  operation  in  the  United  States  and  British  Provinces  make 
a  total  of  about  45,000  miles,  on  nine-tenths  of  which  Morse's  apparatus  is 
used,  House's  and  Bain's  being  chiefly  employed  on  the  remainder.  With 
Morse's  instruments  about  9,000  letters  may  be  transmitted  in  an  hour.  The 
construction  of  the  line  costs  not  far  from  $150  a  mile. 

The  same  year  in  which  Morse  perfected  his  invention  (1837),  plans  for 
telegraphic  communication  based  on  the  deflections  of  the  needle  were  an- 
nounced by  Wheatstone  in  England,  and  Steinheil  [stine'-Mle],  &  German 
philosopher,  to  whom  the  discovery  that  the  earth  could  be  made  to  com- 
plete the  circuit  seems  to  be  due.  They  are  therefore  sometimes  mentioned 
as  entitled  to  share  with  Morse  the  honor  of  his  great  invention.  Their  sys- 
tems, however,  were  but  modifications  of  what  had  been  proposed  some  years 
before ;  though  practicable,  they  could  not  compete  in  rapidity  of  operation 
with  Morse's,  and  consequently  never  came  into  general  use. 

923.  ELECTRO-MAGNETIC  CLOCKS. — American  ingenuity 

forth  by  the  discovery  of  electro-magnetism  1  By  whom  and  when  was  the  first  per- 
fect apparatus  for  transmitting  and  recording  communications  invented  ?  What  two 
improvements  prepared  the  way  for  Morse's  invention  ?  How  was  Morse  enabled  to 
test  his  invention  ?  What  was  the  result?  What  was  the  first  news  transmitted? 
How  many  miles  of  telegraph  are  now  in  operation  ?  On  how  much  of  this  is  Morse's 
apparatus  used?  What  is  the  cost  of  constructing  a  telegraphic  line?  Who  aro 
sometimes  mentioned  as  sharing  with  Morse  the  honor  of  inventing  the  telegraph? 


ELECTRO-MAGNETIC  CLOCKS.  365 

has  applied  electro-magnetism  to  the  determining  of  minute 
intervals  of  time  and  the  regulation  of  clocks.  The  time 
of  astronomical  observations  may  thus  be  fixed  with  perfect 
precision  to  the  tenth  of  a  second. 

The  pendulum  of  a  clock,  for  instance,  is,  by  some  mechanical  contrivance, 
made  by  its  vibrations  to  close  and  break  a  galvanic  circuit.  With  Morse's 
apparatus,  each  vibration  is  indicated  by  a  dot  on  a  strip  of  paper  passed  in 
front  of  the  style.  If  now  an  observer  have  a  signal-key  connected  with  the 
same  circuit,  by  depressing  it  the  instant  a  star  passes  one  of  the  wires  of 
his  telescope,  he  permanently  records  its  transit  on  the  same  paper  by  a  dot 
intermediate  between  two  vibration-dots,  the  exact  time  of  which  is  known. 

924.  By  the  same  agency  a  number  of  clocks  may  be 
made  to  keep  uniform  time. 

^  This  is  effected  by  connecting  any  number  of  distant  clocks,  by  means  of 
wires,  with  one  standard  time-piece,  which  is  itself  connected  with  a  gal- 
vanic battery, — so  that  the  circuit  may  be  closed  and  broken  by  all  the  pen- 
dulums simultaneously.  Wheels  connect  the  pendulums  with  the  hands  of 
the  clocks,  which  are  thus  made  to  move  with  perfect  uniformity.  Some 
railroad  companies  use  an  arrangement  of  this  kind  to  make  the  clocks  at 
their  different  stations  keep  time  together. 

925.  ELECTRO-MAGNETIC  FIRE-ALARM. — The  principle  of 
the  telegraph  has  been  used  for  raising  a  simultaneous  alarm 
of  fire  at  a  number  of  different  stations  connected  with  one 
principal  station  by  wires.     By  completing  and  breaking 
the  galvanic  circuit,  an  attendant  who  is  constantly  on  watch 
at  the  principal  station,  and  receives  his  information  by  tel- 
egraphic signals  from  the  district  in  which  the  fire  is  de- 
tected, strikes  alarm-bells  at  the  various  distant  stations  a 
certain  number  of  times,  according  to  the  number  of  the 
district  in  question.     Such  an  arrangement  has  been  used 
in  Boston  with  great  success. 

926.  THE  HELIX,  A  MAGNET. — The  helix,  when  traversed 
by  a  current  of  electricity,  not  only  has  high  magnetizing 
powers,  as  we  have  seen,  but  is  also  itself  a  magnet.    If 

What  is  said  of  their  claims  ?  923.  To  what  has  American  ingenuity  applied  electro- 
magnetism  ?  Show  how  an  astronomical  observation  may  be  telegraphically  record- 
ed. 924.  How  may  a  number  of  clocks  be  made  to  keep  uniform  time  by  means  of 
electro-magnetism  ?  925.  For  what  has  the  principle  of  the  telegraph  been  used  ? 
Show  how  an  alarm  of  fire  may  be  simultaneously  raised  at  different  stations. 
926.  What  is  the  effect  of  an  electric  current  traversing  a  helix  on  the  helix  itself? 


360  MAGNETTO-ELECTRICITT. 

suspended  so  as  to  allow  it  freedom  of  motion,  it  points 
north  and  south,  and  dips  like  the  magnetic  needle.  So, 
like  poles  of  two  helices  repel  each  other ;  unlike  poles  at- 
tract each  other. 

Even  when  not  bent  in  the  form  of  helices,  two  wires  traversed  by  elec- 
tric currents,  if  brought  near  each  other  in  parallel  lines  and  free  to  move, 
exhibit  mutual  attraction  or  repulsion.  When  their  currents  move  in  the 
same  direction,  they  attract  each  other  j  when  in  contrary  directions,  they 
repel  each  other. 

Magneto-electricity. 

927.  Not  only  is  magnetism  developed  by  electric  cur- 
rents, but  electric  currents  are  produced  by  magnetism. 
That  branch  of  science  which  treats  of  electric  currents  so 
produced  is  called  Magneto-electricity. 

The  phenomena  of  magneto-electricity,  like  those  of  electro-magnetism, 
go  far  towards  proving  the  intimate  connection  between  electricity  and  mag- 
netism, if  not  their  actual  identity. 

928.  Experiments. — Connect  the  ends  of  wire  from  a  helix  with  a  galva- 
nometer. Then  quickly  thrust  into  the  helix  one  of  the  poles  of  a  bar  mag- 
net. The  needle  of  the  galvanometer  is  at  once  deflected,  showing  the  pas- 
sage of  an  electric  current  over  the  wire.  If  the  opposite  pole  is  introduced 
into  the  helix,  a  current  passes  in  the  contrary  direction. 

Within  a  helix  place  ft  soft  iron  bar  of  such  length  that  each  end  may 
project  a  little.  Over  its  ends  bring  the  poles  of  a  horse-shoe  magnet,  so 
suspended  as  to  have  freedom  of  revolution.  On  turning  the  magnet  rapidly, 
the  poles  of  the  bar  are  reversed  twice  for  each  revolution,  and  an  electric 
current  is  produced  on  the  wire,  as  is  shown  by  a  galvanometer  attached  to 
it.  This  principle  has  been  applied  in  different  magneto-electric  machines, 
with  which  water  may  be  decomposed,  platinum  wire  heated  to  redness, 
sparks  produced,  shocks  given,  and  other  experiments  performed. 

929.  THE  MAGNETO-ELECTRIC  MACHINE. — Fig.  328  rep- 
resents one  form  of  the  Magneto-electric  Machine. 

S  is  a  compound  horse-shoe  magnet  supported  on  three  pillars.  In  front 
of  its  poles,  and  as  near  as  it  can  be  brought  without  touching,  is  a  bar  of 
soft  iron  bent  at  right  angles,  and  surrounded  with  several  coils  of  insulated 
copper  wire.  The  ends  of  this  wire  are  pressed  by  springs  against  a  con- 
Prove  that  it  renders  the  helix  magnetic.  What  phenomena  are  exhibited  by  two 
straight  wires  traversed  by  electric  currents,  when  brought  near  each  other  ? 
92T.  What  is  Magneto-electricity  ?  What  is  said  of  its  phenomena  ?  92S.  What  is  the 
first  experiment  illustrative  of  magneto-electricity  ?  The  second  experiment  ?  In 
what  is  the  principle  here  described  applied  ?  929.  Describe  the  Magneto-electric 


jilAGNETO-ELECTEIC  MACHINE. 

Fig.  828. 


367 


MAGNETO-ELECTRIC  MACHINE. 

ducting  metallic  plate,  connected  by  wires  passing  under  the  stand  with  the 
screw-cups  A,  B.  The  soft  iron  armature  just  described  is  mounted  on  an 
axis  which  is  made  to  revolve  by  a  wheel  turned  by  a  handle.  The  handle 
being  rapidly  turned,  each  half-revolution  of  the  armature  brings  its  extrem- 
ities near  opposite  poles  of  the  magnet,  thus  reversing  its  polarity,  and  pro- 
ducing a  strong  electric  current  on  the  wire.  If  small  copper  cylinders 
attached  to  the  wires  are  grasped  one  in  each  hand,  as  shown  in  the  figure, 
a  series  of  severe  shocks  are  received,  and  the  muscles  are  so  contracted  that 
it  is  almost  impossible  to  open  the  hands  and  let  go  the  conductors. 

Machines  of  this  kind,  adapted  to  medical  use,  have  been  found  effica- 
cious in  cases  of  rheumatism,  dyspepsia,  sprains,  nervous  diseases,  &c.,  the 
current  being  made  to  pass  through  the  diseased  part. 

IHamagnetism. 

930.  Experiments  with  powerful  electro-magnets  show 
that  almost  all  substances  are  susceptible  of  magnetic  in- 
fluence. Some  are  attracted  by  the  magnet ;  others,  re- 
pelled ;  while  a  few  are  not  acted  on  at  all,  though  when 
more  powerful  magnets  shall  be  made  they  may  perhaps 
be  found  to  fall  under  one  of  the  two  previous  classes. 

Hence  arises  a  three-fold  division  of  bodies.  1.  Mag- 
netic bodies,  or  such  as  are  attracted  by  an  electro-magnet. 

Machine  represented  in  Fig.  323,  and  its  mode  of  operation.  What  is  the  effect  of 
such  a  machine  on  the  human  system  ?  What  use  has  been  made  of  machines  of  this 
kind?  930.  What  has  been  shown  by  experiments  with  powerful  electro-magnets? 
Name  the  three  classes  into  which  bodies  are  divided  with  reference  to  the  influence 


368 


DIAMAGNETISM. 


2.  Diamagnetic,  or  such  as  are  repelled, 
such  as  are  not  acted  on  at  all. 

Fig.  329. 


3.  Indifferent,  or 


The  difference  between  these  three  classes 
of  bodies  may  be  illustrated  with  the  apparatus 
shown  in  Fig.  329.  N,  S,  are  the  poles  of  an 
electro-magnet,  which  is  connected  by  the  wires 
C,  Z,  with  a  galvanic  battery.  A  bar  of  iron, 
nickel,  cobalt,  manganese,  or  other  magnetic 
substance,  suspended  between  the  poles  so  as  to 
move  freely,  will  come  to  rest  with  its  ends  as 
near  them  as  possible,  in  the  position  1 1.  On 
25  the  contrary,  a  bar  of  bismuth,  phosphorus,  zinc, 
tin,  or  other  diamagnetic  substance,  similarly 
suspended,  will  be  repelled  and  come  to  rest  at  right  angles  to  the  position 
just  described,  as  shown  by  the  dotted  line, — with  its  sides  opposite  the  poles 
of  the  axis  and  its  ends  as  far  from  them  as  possible.  Similar  attraction  and 
repulsion  are  exhibited  if  the  substances  are  presented  to  either  pole  sepa- 
i  ately.  An  indifferent  substance  will  remain  in  any  position  in  which  it  is 
placed,  being  neither  attracted  like  the  iron  nor  repelled  like  the  bismuth. 

Similar  experiments  may  be  made  on  liquids  and  gases  by  enclosing  them 
in  tubes.  It  is  thus  found  that  oxygen  is  magnetic ;  water,  alcohol,  ether, 
and  the  oils,  diamagnetic. 


CHAPTER  XVIII. 

A  S  T  R  O  N  O  M'Y. 

931.  ASTRONOMY  is  the  science  that  treats  of  the  heav- 
enly bodies, — their  motions,  size,  distance,  &c. 

By  the  heavenly  bodies  are  meant  the  sun,  the  moon, 
stars,  planets,  and  comets. 

932.  Astronomy,  as  it  is  the  most  sublime,  is  also  the  oldest  of  sciences. 
The  shepherds  of  the  patriarchal  age,  tending  their  flocks  by  day  and  night 
beneath  the  canopy  of  heaven,  naturally  directed  their  gaze  to  the  brilliant 


•xerted  on  them  by  electro-magnets.  Define  each.  Illustrate  the  difference  be- 
tween these  three  classes  with  the  apparatus  represented  in  Fig.  329.  How  may  sim- 
ilar experiments  be  made  on  liquids  and  gases  ?  What  gas  is  found  to  be  magnetic? 
What  liquids  are  diamagnetic  ? 

931.  What  is  Astronomy?    What  are  meant  by  the  heavenly  bodies  ?    932.  Who 


ASTRONOMY.  369 

orbs  with  which  it  is  studded,  observed  their  motions,  and  thus  became  the 
first  astronomers.  Chaldean  observations  are  said  to  extend  back  to  within 
a  hundred  years  of  the  flood.  The  Chinese,  also,  paid  great  attention  to  this 
science  in  remote  antiquity.  We  are  told  that  more  than  2,000  years  before 
the  birth  of  Christ,  an  emperor  of  China  put  to  death  his  two  chief  astrono- 
mers for  not  predicting  an  eclipse  of  the  sun. 

Destitute  of  the  admirable  instruments  which  modern  science  has  pro- 
duced and  used  with  signal  success,  the  ancient  astronomers  of  course  erred 
in  many  of  their  conclusions.  We  can  only  wonder  that  they  obtained  as 
much  knowledge  as  they  did  respecting  the  heavenly  bodies. 

933.  To  unfold  the  principles  of  astronomy  at  length 
would  require  a  volume,  and  to  understand  them  thorough- 
ly, a  knowledge  of  the  higher  mathematics  is  essential.   We 
can  here  present  only  such  leading  facts  as  will  serve  to 
give  a  general  view  of  the  science. 

934.  FUNDAMENTAL  FACTS. — The  great  facts  established 
by  the  researches  of  astronomers  are  as  follows  : — 

1.  Space  is  filled  with  worlds. 

Looking  up  into  the  heavens  on  a  clear  night,  we  see  them  all  around  us. 
The  telescope  reveals  millions.  There  are  no  doubt  millions  more  too  remote 
to  be  seen  at  all,  and  others  which  from  being  non -luminous  escape  our  vis- 
ion. Powerful  instruments  reach  to  points  from  which  light,  travelling  as  it 
does  with  the  enormous  velocity  of  192,000  miles  in  a  second,  would  be  60,000 
years  in  reaching  us,  and  throughout  the  whole  of  this  vast  field  worlds  are 
everywhere  scattered.  We  can  but  infer  that  the  regions  to  which  man's  eye 
has  never  penetrated  are  similarly  studded ;  and  that,  if  an  observer  could 
be  transported  to  the  remotest  star  visible  with  his  telescope,  he  would  see 
spread  before  him  in  the  same  direction  a  firmament  no  less  rich  and  splendid 
than  that  which  he  beheld  from  the  earth. 

2.  These  worlds  are  divided  into  systems,  the  members 
of  which  are  bound  together  by  mutual  attraction.     Each 
system  has  a  central  sun,  round  which  the  other  members, 
called  Planets,  revolve.     While  this  revolution  is  going  on, 
the  suns  themselves  with  their  respective  planets  move 
about  a  common  fixed  central  point. 

3.  The  stars  that  we  see  twinkling  in  the  sky  are  suns. 

were  the  first  astronomers  ?  How  far  back  are  Chaldean  observations  said  to  extend? 
What  story  shows  the  attention  paid  to  astronomy  by  the  Chinese  in  remote  anti- 
quity? What  is  said  of  the  ancient  astronomers  ?  What  is  the  first  great  fact  estab- 
lished by  astronomers?  What  facts  are  stated  respecting  the  number  of  worlds? 
What  inference  is  drawn  respecting  the  regions  of  space  impenetrated  by  the  eye  of 
man  ?  How  are  these  worlds  divided  ?  What  are  the  stars  that  wo  see  twinkling 

16* 


370  ASTRONOMY. 

The  planets  that  we  suppose  to  revolve  about  them  are 
non-luminous,  and  therefore  invisible. 

4.  Some  of  these  planets  have  satellites  or  moons  moving 
around  them,  and  with  them  around  the  sun  of  the  system 
to  which  they  belong. 

5.  The  Earth,  which  we  inhabit,  is  a  planet  belonging 
to  what  is  known  as  the  Solar  System,  of  which  the  Sun  is 
the  centre.     The  Earth  is  attended  by  one  satellite  known 
as  the  Moon. 

The  Solar  System. 

935.  The  Solar  System,  as  at  present  known,  consists  of 
the  sun,  its  centre ;  seventy  planets  revolving  round  it, 
of  which  sixty-two,  on  account  of  their  small  size,  are  called 
Asteroids  (starlike  bodies) ;  twenty  moons  revolving  round 
the  planets  ;  and  many  thousand  comets,  the  exact  number 
of  which  is  unknown. 

936.  That  the  earth  and  other  planets  move  round  the  sun,  was  taught  by 
the  philosopher  Pythagoras  about  500  B.  c.  Deceived  by  appearances,  how- 
ever, the  ancients  generally  rejected  this  theory,  and  believed  the  earth  to  be 
the  fixed  centre  of  motion  for  all  the  heavenly  bodies.  Some  made  the  plan- 
ets revolve  round  the  sun,  and  the  sun  carrying  the  planets  with  it  to  move 
round  the  earth.  The  Egyptian  astronomer  Ptolemy  supposed  the  universe 
to  consist  of  a  number  of  hollow  spheres  arranged  one  within  another,  and 
appropriated  respectively  to  the  sun,  the  moon,  the  planets,  and  the  stars. 
The  earth,  according  to  Ptolemy,  was  at  the  centre  of  these  spheres,  which 
turned  round  it  from  east  to  west  every  twenty-four  hours,  carrying  the  stars 
and  planets  with  them ;  being  of  crystal,  they  were  perfectly  transparent, 
and  the  inner  ones  did  not  therefore  obscure  the  more  distant  luminaries 
seen  through  them. 

These  theories,  particularly  Ptolemy's,  prevailed  till  about  the  middle  of 
the  sixteenth  century,  when  the  Prussian  philosopher  Copernicus  revived  the 
teachings  of  Pythagoras,  and  established  what  is  called  from  him  the  Coper- 
nican  System,  which  is  now  acknowledged  as  the  true  theory  of  the  universe. 
Fearing  the  prejudices  of  his  fellow-men,  Copernicus  withheld  his  system 
from  them  for  some  years.  His  great  work,  in  which  his  views  were  embod- 


in  the  sky  ?  Why  are  not  their  planets  visible  ?  By  what  are  some  of  the  planets 
attended  ?  What  is  the  Earth  ?  By  what  is  it  attended  ?  935.  Of  what  does  the 
Solar  System,  as  at  present  known,  consist  ?  936.  What  was  Pythagoras\s  theory  of 
the  universe  ?  What  was  the  belief  of  the  ancients  generally  ?  Give  an  account  of 
Ptolemy's  theory.  By  whom  and  when  was  the  true  system  revived  ?  When  wa» 


THE   SOLAS  SYSTEM.  371 

led,  was  finally  published  in  1543,  just  in  time  for  a  copy  to  be  placed  in  his 
hands  on  his  death-bed. 

The  Copernican  system  at  first  met  with  but  moderate  favor.  Its  truth, 
however,  was  established  by  Galileo,  whose  observations  with  the  newly- 
invented  telescope  afforded  him  incontrovertible  arguments  in  its  favor.  Yet 
the  advocates  of  the  old  system  were  determined  to  close  their  eyes.  On 
Galileo's  announcing  the  discovery  of  four  moons  about  the  planet  Jupiter, 
they  denied  the  possibility  of  their  existence ;  and  when  he  urged  them  to 
look  for  themselves  through  his  telescope,  they  refused  to  have  anything  to 
do  with  an  instrument  they  despised.  An  astronomer  of  Florence  gravely 
argued  that  as  there  were  only  seven  apertures  in  the  head — two  eyes,  two 
ears,  two  nostrils,  and  one  mouth — and  as  there  were  only  seven  metals,  and 
seven  days  in  the  week,  so  there  could  be  only  seven  planets.  As  there  were 
six  principal  planets  and  one  moon  then  known,  the  number  was  complete, 
and  Galileo's  pretended  planets  must  be  impossibilities. — But  such  absurd 
arguments  could  not  long  obscure  the  light  of  truth. 

937.  THE  SUN  (0). — The  Sun,  the  great  source  of  light 
and  heat  to  the  planets,  is  the  centre  of  the  solar  system.    It 
is  an  immense  globe,  five  hundred  times  as  large  as  all  its  plan- 
ets put  together.  Its  diameter  is  882,000  miles.  Placed  where 
the  earth  is,  it  would  fill  the  whole  orbit  of  the  moon,  and 
extend  200,000  miles  beyond  it  in  all  directions.   Its  volume 
is  nearly  a  million  and  a  half  times  as  great  as  the  earth's, 
and  it  contains  more  than  350,000  times  as  much  matter. 

938.  Solar  Spots. — Viewed  through  a  telescope,  the  sun 
looks  like  a  globe  of  fire.     Its  surface,  however,  is  not  al- 
ways wholly  luminous.    A  number  of  dark  spots,  surround- 
ed by  a  lighter  shadow,  are  at  times  scattered  here  and 
there  within  a  zone  extending  35  degrees  on  each  side  of 
the  solar  equator.    The  number  and  size  of  these  spots 
differ  at  different  times ;  for,  while  some  last  a  couple  of 
months  or  even  longer,  others  change  their  form  from  day 
to  day.    They  have  been  known  to  vanish  almost  instantly 
and  to  appear  as  suddenly.    Some  years  none  at  all  are 
visible ;  in  others,  as  many  as  200  are  seen  at  once,  cover- 

the  work  of  Copernicus  relating  to  this  subject  published  ?  By  whom  was  the  truth  of 
the  Copernican  system  established  ?  What  were  the  arguments  with  which  Galileo 
was  met?  937.  What  is  the  Sun  ?  How  great  is  its  diameter  ?  Placed  where  the  earth 
is,  how  far  would  it  extend  ?  How  does  its  volume  compare  with  the  earth's  ?  It* 
matter  ?  988.  How  does  the  sun  look,  when  viewed  through  a  telescope  ?  Describe 
the  spots  which  are  sometimes  visible.  What  is  said  of  their  number  and  size  ?  What 


372  ASTRONOMY. 

ing  so  much  of  the  surface  as  materially  to  diminish  the 
quantity  of  light  emitted. 

By  comparing  a  number  of  observations  on  the  solar  spots,  we  find  that 
they  are  subject  to  periodical  increase  and  decrease.  They  become  larger 
and  more  numerous  for  a  certain  time  till  they  reach  a  maximum,  after  which 
they  gradually  diminish,  till  all  disappear,  or  nearly  so ;  new  ones  then  be- 
come visible,  and  go  on  increasing  during  the  same  period  as  before.  This 
period  seems  to  be  a  little  over  eleven  years. 

Spots  have  occasionally  appeared  of  such  size  that  they  could  be  readily 
discerned  with  the  naked  eye.  One  thus  seen  for  a  week  in  June,  1843,  must 
have  been  77,000  miles  across,  or  nearly  ten  times  the  size  of  the  earth. 

Astronomers  have  tried  to  account  for  the  solar  spots  in  various  ways. 
The  prevailing  opinion  is  that  the  light  received  from  the  sun  does  not  come 
from  its  surface,  but  from  a  luminous  atmosphere  of  great  depth  with  which 
it  is  surrounded ;  and  that  the  spots  in  question  are  simply  portions  of  the 
dark  body  of  the  sun,  which  become  visible  when  the  luminous  atmosphere 
is  opened  by  upward  currents  from  the  surface  or  any  other  agency.  The 
disturbance  of  this  atmosphere,  by  whatever  it  is  caused,  is  most  frequent 
near  the  solar  equator. — Peculiarly  bright  streaks  of  light,  called  faculce,  are 
often  found  near  the  spots  or  where  they  have  just  disappeared.  They  are 
supposed  to  be  the  ridges  of  vast  waves  in  the  luminous  atmosphere  just  de- 
scribed. 

939.  Constitution  of  the  Sun. — The   sun's  density  is 
about  one-fourth  that  of  the  earth.    Respecting  its  consti- 
tution little  is  known,  nor  are  we  any  better  informed  as  to 
what  produces  its  intense  heat  and  light.     It  was  formerly 
supposed  that  the  whole  mass  was  in  a  state  of  combustion. 
But  how  can  such  combustion  be  kept  up  without  dimin' 
ishing  the  material  on  which  it  feeds  ?     The  difficulty  of 
answering  this  question  has  led  the  later  astronomers  to 
point  to  friction  or  electricity  as  fche  most  probable  source 
of  solar  heat  and  light. 

940.  JMbtions. — The  more  permanent  of  the  sun's  spots, 
if  observed  from  time  to  time,  are  found  %to  change  their 
position  on  its  disk,  or  face.     First  becoming  visible  on  the 


is  found  by  comparing  a  number  of  observations  on  the  solar  spots?  What  is  the 
length  of  the  period  ?  Of  what  sizo  have  spots  occasionally  appeared  ?  What  was 
the  diameter  of  one  seen  in  June,  1843  ?  What  is  the  prevailing  opinion  of  astrono- 
mers respecting  these  spots  ?  What  are  faculae,t  What  are  they  supposed  to  be? 
939.  How  does  the  sun's  density  compare  with  the  earth's  ?  What  is  known  respect- 
ing its  constitution  and  heat?  To  what  have  the  later  astronomers  pointed  as  th$ 
most  probable  source  of  solar  heat  and  light?  940.  How  is  it  proved  that  the  sun 


THE  ZODIACAL  LIGHT.  373 

east  side,  they  gradually  move  towards  the  west,  and  in 
about  thirteen  days  are  lost  from  sight  in  that  direction. 
After  a  similar  period  they  reappear  in  the  east.  This 
phenomenon  shows  that  the  sun  turns  on  its  axis  from 
west  to  east ;  its  revolution  is  performed  in  about  25  days, 
8  hours. 

Besides  turning  on  its  axis,  the  sun,  attended  by  its 
planets,  moves  at  the  rate  of  8  miles  a  second  in  a  circular 
path  round  a  centre  far  off  in  the  fields  of  space.  So  vast 
is  this  path  that  it  will  take  the  sun  18,200,000  years  to  get 
once  completely  round  it. 

941.  The  Zodiacal  Light. — A  faint  light,  shaped  like  a 
sugar-loaf,  is  sometimes  seen  stretching  obliquely  upward 
in  the  heavens,  from  70  to  100  degrees,  from  that  part  of 
the  horizon  where  the  sun  is  about  rising  or  has  just  set. 
This  phenomenon  is  known  as  the  Zo-di'-a-cal  Light.    It  is 
brightest  and  most  distinctly  defined  in  tropical  regions, 
where  it  is  visible  most  of  the  time.     In  high  latitudes  it  is 
seldom  clearly  seen,  except  during  March  and  April  just 
after  sun-set,  and  in  September  and  October  immediately 
before  dawn. 

The  cause  of  the  zodiacal  light  is  unknown.  Some  suppose  it  to  be  an 
expansion  of  the  solar  atmosphere  ;  others,  a  thin  vapor,  charged  with  mat- 
ter from  the  tails  of  comets,  of  which  the  sun's  attraction  has  deprived  them ; 
others,  again,  have  suggested  that  it  is  a  remnant  of  the  original  matter  of 
which  both  sun  and  planets  were  made.  The  latest  theory  is,  that  it  is  a  neb- 
ulous ring,  surrounding  the  Earth,  like  the  ring  of  the  planet  Saturn. 

The  Planets. 

942.  By  the  Planets  of  the  solar  system  are  meant  those 
heavenly  bodies  that  revolve  directly  about  the  sun  in  ob- 
long curves,  and  shine  by  its  reflected  light. 

The  wordplanetes  in  Greek  means  "  a  wanderer",  and  the  bodies  in  ques- 
tion are  so  called  in  contradistinction  to  the  fixed  stars,  which  keep  the  same 

turns  on  its  axis?  What  is  the  time  of  its  revolution ?  What  other  motion  has  the 
»un  ?  How  large  is  the  path  it  travels  ?  941.  Describe  the  Zodiacal  Light  Where 
is  it  brightest  ?  When  is  it  seen  in  high  latitudes  ?  What  opinions  have  been  ad- 
vanced to  account  for  the  zodiacal  light  ?  942.  What  are  the  Planets  ?  What  does 
the  wordjpfcmetesmean?  From  what  are  the  planets  to  be  distinguished?  How 


374  ASTRONOMY. 

position  in  the  heavens  relatively  to  each  other.  The  planets  and  the  fixed 
stars  are  easily  distinguished ;  the  former  shine  with  a  steady  light,  the  latter 
twinkle. 

943.  The  moons  are  sometimes  called  Secondary  Plan- 
ets.    In  that  case,  the  bodies  that  revolve  directly  about 
the  sun  are  called  Primary  Planets. 

944.  The  planets  are  also  distinguished  as  Inferior  and 
Superior.    The  Inferior  Planets  are  those  that  are  nearer 
to  the  sun  than  the  earth  is ;  the  Superior  Planets  are  those 
that  are  farther  from  the  sun  than  the  earth  is. 

945.  ORBITS  OF  THE  PLANETS. — The  path  of  a  planet 
round  the  sun  is  called  its  Orbit.     The  planets  being  at 
different  distances  from  the  sun,  their  orbits  differ  in  length, 
though  they  are  similar  in  shape. 

946.  The  planetary  orbits  are  not  circles,  but  oblong 
curves  called  Ellipses.     Hence  a  planet  is  nearer  the  sun  in 
one  part  of  its  course  than  in  another.    That  point  of  its 
orbit  at  which  it  is  nearest  the  sun  is  called  .its  perihelion 
(plural,  perihelia)  ;  that  in  which  it  is  farthest  from  the  sun 
is  its  aphelion  (plural,  aphelia).     When  a  planet's  distance 

from  the  sun  is  spoken  of,  its  mean  dis- 
tance is  meant.  This  is  obtained  by  add- 
ing its  greatest  and  least  distance  to- 
gether and  dividing  by  2. 

These  definitions  are  illustrated  in  Fig.  330. 
A  B  P  C  represents  an  ellipse.  S  is  the  sun,  situ- 
ated not  at  the  centre  of  the  ellipse,  but  at  one  of  two 
points  within  it  called  foci.  P  shows  the  position 
of  a  planet  at  its  perihelion,  and  A  at  its  aphelion. 

The  orbits  of  the  planets  lie  in  different  planes, 
more  or  less  inclined  to  each  other. 

947.  Besides  their  revolution  round  the  sun,  the  planets 
have  another  motion  round  their  own  axes.     The  time  that 


can  the  planets  and  the  fixed  stars  be  told  apart?  943.  What  constitutes  the  differ- 
ence between  Primary  and  Secondary  Planets  ?  94-4.  Between  Inferior  and  Superior 
Planets  ?  945.  What  is  a  planet's  orbit  ?  946.  What  is  the  shape  of  the  planetary 
orbits  ?  What  is  a  planet's  Perihelion  ?  Aphelion  ?  When  a  planet's  distance  from 
the  sun  is  spoken  of,  what  is  meant  ?  How  is  the  mean  distance  obtained  ?  Illus- 
trate these  definitions  with  Fig.  330.  What  is  said  of  the  planes  of  the  orbits? 
NT.  What  other  motion  have  the  planets  besides  their  revolution  round  the  sun  ? 


THE  PLANETS. 


375 


it  takes  a  planet  to  make  one  revolution  on  its  axis  is  called 
its  Day. 

948.  TABLE. — A  Table  of  the  planets  follows,  in  the  or- 
der of  their  distances  from  the  sun,  which  are  given  in  the 
second  column.  Their  diameters  in  miles  are  given  in  the 
third  column ;  the  number  of  our  days  that  it  takes  them 
to  revolve  round  the  sun,  in  the  fourth  ;  and  the  hours  re- 
quired for  the  revolution  of  each  on  its  axis,  in  the  fifth. 
The  Tables  in  the  Fifth  Edition  of  Herschel's  "  Outlines  of 
Astronomy"  (1858)  are  here  followed. 


Name. 

Distance  from 
Sun  in  miles. 

Diameter 
in  miles. 

Year  expressed  in 
the  Earth's  days. 

Day  expressed 
in  hours,  &c. 

Mercury    . 

36,890,000 

3,183 

88 

21h     5m 

Yenus  .    . 

68,770,000 

8,108 

225 

23    21 

Earth    .     . 

95,298,260 

7,926 

365'A 

24 

Mars     .    . 

145,205,000 

4,546 

687 

24  37 

ASTEROIDS 

j  from  210  to 

est.  at  from 

from  1,191  ) 

unknown 

(62) 

1  301  millions 

100  to  1,000 

to       2,051  ) 

Jupiter 

495,815,500 

90,734 

4,333 

&  55«  27' 

Saturn  .    . 

909,029,700 

76,791 

10,759 

10  29    17 

Uranus 

1,828,048,000 

35,307 

30,687 

9   30 

Neptune    . 

2,862,404,000 

39,793 

60,126 

unknown 

949.  Mercury,  Venus,  Mars,  Jupiter,  and  Saturn,  being  visible  to  the 
naked  eye,  were  known  to  the  ancients.  Uranus  was  discovered  in  1781  by 
Sir  William  Herschel,  from  whom  it  was  first  commonly  called  Herschel.  Its 
discoverer  gave  it  the  name  of  Georgium  Sidus,  in  honor  of  King  George  III. 
Both  these  names,  however,  were  discarded  for  the  mythological  one  by 
which  it  is  at  present  known.  The  first  of  the  asteroids,  Ceres,  was  discov- 
ered in  1801  by  the  Sicilian  astronomer  Piazzi  [pe-at'-ze].  Pallas  was  added 
to  the  list  in  1804 ;  Juno,  in  1804 ;  Vesta,  in  1807 ;  and  the  remainder,  since 
1844. 

Neptune  was  discovered  in  1846  by  Dr.  Galle  [gal'-la],  of  Berlin.  It  was 
first  called  Le  Verrier  [luTi  va-re-a'],  in  honor  of  an  eminent  French  astrono- 


"WTiat  is  meant  by  a  planet's  day  ?  948.  Eeferring  to  the  Table,  which  of  the  planets 
do  you  find  the  smallest  (the  asteroids  excepted),  and  which  the  largest?  Which 
takes  the  shortest  time  to  revolve  around  the  sun,  and  which  the  longest  ?  Which 
three  have  a  day  very  nearly  as  long  as  the  Earth's  ?  Which  three  have  days  less  than 
half  as  long  as  the  Earth's?  949.  Which  of  the  planets  were  known  to  the  ancients? 
Which  was  the  next  discovered  ?  What  other  names  has  Uranus  borne  ?  When  and 
by  whom  was  the  first  asteroid  discovered  ?  When  were  the  rest  added  to  the  list  ? 
When  and  by  whom  was  Neptune  discovered  ?  What  was  it  first  called,  and  why  ? 


376  ASTRONOMY. 

mer,  who  by  a  series  of  calculations  established  the  fact  that  there  was  a 
more  distant  planet  than  Uranus,  and  instructed  Dr.  Galle  in  what  part  of  the 
Leavens  to  look  for  it. 

950.  BODE'S  LAW. — By  comparing  the  distances  of  the 
planets  from  the  sun,  Bode  [bo'-da]  arrived  at  the  following 
law : — Take  the  geometrical  progression 

0     3     6     12     24     48     96     192     384, 
each  term  of  which  (after  the  second)  is  obtained  by  doub- 
ling the  preceding  one.     To  each  term  add  4,  and  we  get 

4  7  10  16  28  52  100  196  388. 
The  distances  of  the  nearer  planets  are  approximately  pro- 
portioned to  these  numbers.  That  is,  Mercury's  distance  be- 
ing 36,890,000  miles,  Venus's  will  be  J  as  much,  the  Earth's 
Y> ;  <fcc.  Bode's  law,  however,  does  not  apply  to  Saturn, 
Uranus,  and  Neptune.  They  are  all,  particularly  the  last, 
much  nearer  the  sun  than  this  law  would  make  them. 

951.  Fig.  331  shows  the  comparative  size  of  the  planets. 
The  asteroids  are  too  small  to  appear  on  this  'scale. 

Fig.  881. 

,:::         I  f  f       * 


r 


Herschel  uses  the  following  illustration  to  give  an  idea  of  the  relative  size 
of  the  planets  and  their  orbits  : — "  Choose  any  well  levelled  field  or  bowling- 
green.  On  it  place  a  globe  two  feet  in  diameter ;  this  will  represent  the  Sun 
Mercury  will  be  represented  by  a  grain  of  mustard-seed,  on  the  circumference 
of  a  circle  164  feet  in  diameter  for  its  orbit ;  Venus  a  pea,  on  a  circle  of  284 
feet  in  diameter ;  the  Earth  also  a  pea,  on  a  circle  of  430  feet ;  Mars  a  rather 
large  pin's  head,  on  a  circle  of  654  feet ;  the  Asteroids,  grains  of  sand,  in 
orbits  of  from  1000  to  1200  feet ;  Jupiter,  a  moderate-sized  orange,  in  a  circle 

>50.  State  Bode's  Law.    951.  What  does  Fig.  331  show  ?    Kepeat  the  illustration  used 


KEPLER'S  LAWS. 


377 


nearly  half  a  mile  across ;  Saturn  a  small  orange,  on  a  circle  of  four-fifth? 
of  a  mile ;  Uranus  a  full-sized  cherry,  or  small  plum,  upon  the  circumference 
of  a  circle  more  than  a  mile  and  a  half;  and  Neptune  a  good-sized  plum, 
on  a  circle  about  two  miles  and  a  half  in  diameter." 

952.  KEPLER'S  LAWS. — The  laws  that  regulate  the  plan- 
etary motions  were  unknown  till  the  commencement  of  the 
seventeenth  century,  when,  after  a  long  and  careful  com- 
parison of  numerous  observations,  they  were  discovered  by 
John  Kepler,  a  celebrated  German  astronomer,  who  thus 
won  the  title  of  "  the  Legislator  of  the  Heavens".    Kep- 
ler's laws  apply  to  the  moons  in  their  revolutions  about 
their  primary  planets,  as  well  as  to  the  latter. 

953.  Kepler's  First  Law. — The  orbits  of  the  planets  are 
ellipses  having  one  focus  in  common,  and  in  this  common 
focus  the  sun  is  situated. 

The  principal  forces  acting  on  the  planets  are  the  sun's  attraction  and  the 
original  force  of  projection.  These  forces  alone  would  cause  each  planet  to 
move  about  the  sun  in  a  perfect  ellipse.  The  attraction  of  the  other  heavenly 
bodies,  however,  produces  Perturbations,  as  they  are  called,  and  thus  each 
orbit  constantly  deviates  in  a  slight  degree  from  an  ellipse. 

The  ellipses  described  by  the  planets  differ  from  circles  in  different  de- 
grees. The  orbits  of  Mercury  and  several  of  the  asteroids  deviate  most ; 
those  of  Neptune  and  Venus  are  nearly  circular. 

954.  Kepler* s  Second  Law. — The 
Radius  Vector  of  a  planet  passes  over 
equal  areas  in  equal  times. 

The  Radius  Vector  is  a  line  con- 
necting  the  centre  of  a  planet,  as  it 
traverses  its  orbit,  with  the  centre 
of  the  sun. 

Thus,  in  Fig.  332,  the  lines  S  A,  SB,  SC, 
Ac.,  represent  the  radius  vector  of  the  planet 
there  traversing  its  elliptical  orbit.  The  whole 
space  included  within  the  orbit  is  divided  into 
12  equal  triangles,  1,  2,  3,  4,  &c. ;  and  these, 

by  Herschel  to  give  an  idea  of  the  relative  size  of  the  planets.  952.  When  were  th« 
laws  that  regulate  the  planetary  motions  first  known  ?  By  whom  were  they  discov- 
ered ?  To  what  do  Kepler's  Laws  apply  ?  953.  Repeat  Kepler's  First  Law.  How  is 
the  elliptical  form  of  the  orbits  accounted  for  ?  What  is  said  of  the  ellipses  described 
by  the  planets?  Which  of  the  orbits  deviate  most  from  a  circle?  Which  deviate 
very  little?  954.  What  is  Kepler's  Second  Law  ?  What  is  the  Radius  Vector  ?  Illus- 


V 


378  ASTRONOMY. 

according  to  the  law  just  stated,  must  be  traversed  by  the  radius  vector  in 
equal  times. 

It  follows  from  this  law  that  the  velocity  of  a  planet  differs  at  different 
points  of  its  orbit,  being  greatest  at  its  perihelion,  and  least  at  its  aphelion. 
A  B.  C  D,  and  the  other  arcs  that  form  the  bases  of  the  twelve  triangles, 
differ  in  length,  but  have  to  be  traversed  in  the  same  time.  The  planet  must 
therefore  move  fastest  over  the  longest  arcs,  which  are  at  its  perihelion,  and 
slowest  over  the  shortest  arcs,  which  are  at  its  aphelion.  In  going  from  its 
aphelion  to  its  perihelion,  the  arcs  keep  increasing,  and  the  velocity  of  the 
planet  is  accelerated ;  from  its  perihelion  to  its  aphelion,  the  arcs  keep  di- 
minishing, and  the  velocity  of  the  planet  is  retarded.  Yet  in  going  the  whole 
distance  from  its  aphelion  to  its  perihelion  a  planet  takes  precisely  the  sam« 
time  as  in  performing  the  opposite  half  of  its  course. 

The  cause  of  this  difference  of  velocity  is  easily  explained.  In  travelling 
towards  its  perihelion,  a  planet  is  constantly  acted  on  by  the  sun's  attraction 
in  the  same  general  direction  as  that  in  which  it  is  moving,  and  this  attrac- 
tion becomes  stronger  and  stronger  as  it  approaches  the  sun.  When  return- 
ing to  its  aphelion,  on  the  contrary,  it  is  acted  on  by  the  sun's  attraction  in 
a  direction  opposite  to  that  in  which  it  is  moving. 

955.  JZepler^s  Third  Law. — The  squares  of  the  planets* 
times  of  revolution  round  the  sun  are  proportioned  to  the 
cubes  of  their  distances  from  the  latter. 

For  example,  Mercury's  year  consists  of  88  days,  Venus's  of  225  days ; 
Mercury  is  86,890,000  miles  from  the  sun,  and  Venus  68,770,000.  Then  the 
following  proportion  holds  good,  or  nearly  so  : — 

(88)"  :  (225)2  :  :  (36,890,000)*  :  (68,770,000)' 

956.  Kepler's  laws  have  been  verified  by  all  the  observations  made  since 
his  time.  They  gave  a  wonderful  impetus  to  the  science,  corrected  many 
false  notions,  and  enabled  astronomers  to  arrive  at  new  facts  from  facts  al- 
ready known.  After  many  attempts  and  failures,  the  third  law  was  finally 
reached  on  the  8th  of  May,  1618.  "Perhaps",  says  Playfair,  "philosophers 
will  agree  that  there  are  few  days  in  the  scientific  history  of  the  world  which 
deserve  so  well  to  be  remembered." 

957.  ASPECTS  OF  THE  PLANETS. — By  the  Aspects  of  the 
planets  are  meant  their  positions  in  their  orbits  relatively 
to  each  other.  The  aspects  most  frequently  alluded  to  are 
Quadrature,  Conjunction,  and  Opposition. 

trate  this  law  with  Fig.  332.  "What  follows  from  this  law  with  respect  to  the  velocity 
of  a  planet  ?  In  what  part  of  its  orbit  does  a  planet  move  with  accelerated  velocity  ? 
In  what,  with  retarded?  Show  the  difference  in  the  case  of  the  Earth.  "What  is  the 
cause  of  this  difference  of  velocity?  955.  State  Kepler's  Third  Law,  and  give  an  ex- 
ample. 956.  "What  is  said  of  Kepler's  Laws  and  the  estimation  in  which  the  third  is 
6eld  by  philosophers?  957.  What  is  meant  by  the  Aspects  of  the  planets  ?  What 


ASPECTS   OP  THE  PLANETS. 


379 


958.  Quadrature. — Two  heavenly  bodies  are  said  to  be 
in  Quadrature  when  they  are  90  degrees  apart ;  that  is, 
when,  if  either  were  placed  on  the  other's  orbit  at  a  point 
corresponding  to  its  position  on  its  own,  the  arc  between 
them  would  subtend  an  angle 

of  90°  at  the  focus.  Thus, 
in  Fig.  333,  E  represents  the 
Earth,  and  Q  Mars  in  quad- 
rature. In  almanacs  and 
astronomical  works,  quad- 
rature is  denoted  by  the 
sign  D. 

959.  Conjunction. — Hea- 
venly bodies  are  said  to  be  in 
Conjunction  when  they  are 
seen  in  the  same  quarter  of 
the  heavens.    Thus,  in  Fig. 
333,  Venus  (V),  the  Sun  (S), 

and  Mars  (N),  are  in  conjunction,  being  in  the  same  direc- 
tion from  the  Earth  (E).  Conjunction  is  denoted  by  the 
sign  6 . 

Conjunctions  are  of  two  kinds,  Superior  and  Inferior.  A  planet  is  in  Su- 
perior Conjunction  when  it  is  in  conjunction  on  the  opposite  side  of  the  sun 
from  the  Earth,  as  Venus  at  W,  and  Mars  at  N.  A  planet  is  in  Inferior  Con- 
junction when  it  is  in  conjunction  on  the  same  side  of  the  Sun  as  the  Earth  is, 
as  Venus  at  V.  It  is  evident  that  the  superior  planets  can  never  be  in  infe- 
rior conjunction. 

960.  Opposition. — Two  heavenly  bodies  are  said  to  be 
in  Opposition  when  they  are  in  directly  opposite  quarters 
of  the  heavens.     Thus,  in  Fig.  333,  Mars  at  M  and  the  Sun 
(S)  are  in  opposition,  because  relatively  to  the  Earth  (E) 
they  lie  in  opposite  directions.     The  inferior  planets  never 
appear  in  opposition.    Opposition  is  denoted  by  the  sign  8 . 

are  the  three  principal  aspects  ?  958.  When  are  two  heavenly  bodies  said  to  be  in 
Quadrature  ?  Illustrate  with  the  Figure.  959.  When  are  heavenly  bodies  said  to  be 
in  Conjunction  ?  Illustrate  with  Fig.  333.  How  many  kinds  of  conjunctions  are 
there?  When  is  a  planet  said  to  be  in  Superior  Conjunction  ?  In  Inferior  Conjunc- 
tion? To  what  planets  is  inferior  conjunction  confined?  960.  When  are  two  heav- 
enly bodies  said  to  be  in  Opposition  ?  Illustrate  with  Fig.  833.  961.  What  is  meant 


SSO  ASTRONOMY. 

961.  Transits. — The  passage  of  an  inferior  planet  across 
the  Sun's  disk  is  called  its  Transit.     In  Fig.  333,  Yenus  at 
V  is  making  her  transit. 

A  transit  can  take  place  only  when  a  planet  is  in  inferior  conjunction. 
But,  as  the  orbits  of  the  planets  are  in  different  planes,  there  may  be  inferior 
conjunctions  without  any  transit.  Venus  may  be  seen  from  the  Earth  in  the 
same  quarter  as  the  Sun,  and  yet  lie  out  of  the  plane  which  connects  the  cen- 
tres of  the  Sun  and  the  Earth. 

962.  Occultation. — When  a  planet  or  star  is  hid  from 
the  view  of  an  observer  on  the  Earth  by  the  interposition 
of  some  other  heavenly  body,  it  is  said  to  suffer  occupation. 

963.  REAL  AND  APPARENT  MOTIONS. — An  observer  at 
the  Sun  would  see  all  the  planets  moving  around  him  from 
west  to  east  with  perfect  regularity  and  always  in  the  same 
direction.     He  would  see  their  Real  Motions.      An  ob- 
server on  the  Earth  sees  only  their  Apparent  Motions,  and 
these  are  so  irregular  that   one  might   almost  fancy  the 
bodies  in  question  wandering  through  space  without  any 
fixed  law  to  direct  their  course.     They  are  seen  at  one 
time  moving  from  west  to  east,  at  another  stationary,  and 
again  pursuing  a  retrograde  course  from  east  to  west. 

The  reasons  of  this  are— 1.  "We  are  95,000,000  miles  from  their  centre  of 
motion.  2.  We  are  ourselves  moving,  both  round  the  sun  and  round  the 
Earth's  axis.  Unconscious  of  these  motions,  we  intuitively  attribute  the 
changes  of  direction  produced  by  them  to  the  motions  of  the  orbs  around  us ; 
just  as  a  person  on  a  boat,  when  it  begins  to  move,  seems  to  be  at  rest  him- 
self, and  to  see  the  wharf  receding  from  him. 

964.  ARE  THE  HEAVENLY  BODIES  INHABITED? — This 
question  is  often  asked,  but  can  not  be  answered.     No  evi- 
dences of  inhabitants  have  ever  been  discovered,  even  in 
the  Moon,  which  is  the  nearest  to  us  of  all  the  heavenly 
bodies ;  nor  can  there  be  any  till  great  improvements  have 
been  made  in  the  telescope.     Nothing,  however,  seems  to 
be  created  without  an  object ;  and,  humanly  speaking,  it 
would  be  strange  if  of  all  the  orbs  which  Omnipotence  has 

by  a  Transit?  Show  the  difference  between  a  transit  and  inferior  conjunction. 
962.  When  is  a  planet  or  star  said  to  suffer  occultation  ?  963.  What  is  the  difference 
between  the  Eeal  and  the  Apparent  Motions  of  the  planets?  Describe  the  apparent 
motions.  What  causes  are  assigned  for  their  irregularity  ?  964.  What  is  said  with 


MERCURY.  381 

called  into  being  our  little  world  were  the  only  one  peopled 
by  intelligent  creatures. 

If  the  planets  are  inhabited,  it  must  be  by  creatures  constituted  very  dif- 
ferently from  the  human  race.  Surrounded  by  entirely  different  circum- 
stances as  regards  temperature,  gravity,  atmosphere,  &c.,  the  inhabitants  of 
the  different  planets  must  be  distinct  races  each  from  every  other.  Yet  who 
can  doubt  that  the  same  Infinite  Wisdom  that  has  adapted  us  to  our  sphere 
could  as  easily  adapt  them  to  theirs  ? 

We  proceed  to  consider  the  planets  in  turn.  The  char- 
acter annexed  to  the  name  is  the  mark  by  which  the  planet 
is  denoted. 

965.  MERCURY  (  £  ). — The  nearest  planet  to  the  Sun  is 
Mercury.  Under  favorable  circumstances,  Mercury  may 
be  seen  at  certain  times  of  the  year  for  a  few  minutes  after 
sun-set  or  before  sun-rise.  At  other  times  it  keeps  so  close 
to  the  Sun  as  to  be  invisible,  being  lost  in  the  superior 
brightness  of  his  rays  in  the  daytime,  and  setting  and  rising 
so  nearly  at  the  same  time  with  him  as  to  afford  no  oppor- 
tunity of  observation. 

To  the  naked  eye  Mercury  looks  like  a  star  of  the  third 
magnitude,  twinkling  (unlike  the  other  planets)  with  a  pale 
rosy  light.  Viewed  through  the  telescope,  it  exhibits  sim- 
ilar phases  or  changes  of  appearance  to  those  of  the  moon 
(from  full  to  new)  ;  this  is  because  we  see  more  of  its  en- 
lightened side  at  one  time  than  another. 

The  solar  heat  received  at  Mercury  is  seven  times  as  great  as  that  of  the 
Earth, — a  temperature  more  than  sufficient  to  make  water  boil.  Mercury's 
light  is  also  seven  times  as  intense  as  ours,  and  the  Sun  seen  from  this  planet 
would  look  seven  times  as  large  as  it  does  to  us.  No  permanent  spots  are 
visible  either  on  Mercury  or  Venus,  whence  it  has  been  supposed  that  we  do 
not  see  the  surfaces  of  these  planets,  but  only  their  atmospheres  loaded  with 
clouds,  which  may  serve  to  mitigate  4he  otherwise  intense  glare  of  the  sun. 
A  German  astronomer,  however,  at  the  commencement  of  the  present  cen- 
tury, observed  what  he  regarded  as  a  number  of  mountains  on  the  surface 
of  Mercury,  one  of  which  he  computed  to  be  over  10  miles  in  height. 

respect  to  the  heavenly  bodies'  being  inhabited?  965.  What  is  the  nearest  planet  to 
the  Sun  ?  When  is  Mercury  visible  ?  What  makes  it  invisible  at  other  times  ?  How- 
does  Mercury  look  to  the  naked  eye  ?  Viewed  through  the  telescope,  what  phases 
does  it  present  ?  How  do  tho  solar  heat  and  light  received  at  Mercury  compare  with 
ours  ?  Are  any  permanent  spots  visible  on  Mercury  or  Venus  ?  To  what  supposi- 
tion has  this  fact  led  ?  What  was  observed  on  Mercury  by  a  German  astronomer  ? 


382  ASTRONOMY. 

Mercury's  orbit  deviates  from  a  circle  much  more  than  that  of  any  other 
planet,  the  asteroids  excepted.  This  circumstance,  combined  with  the  incli- 
nation of  its  axis  to  the  plane  of  its  orbit,  must  produce  a  great  variety  of 
seasons,  and  extreme  changes  of  temperature. 

966.  VENUS   (j). — The  second  planet  from  the  Sun  is 
Venus.     On  account  of  its  nearness,  it  appears  larger  and 
more  beautiful  to  us  than  any  other  member  of  our  plane- 
tary system.     So  bright  is  Venus  that  it  is  sometimes  visi- 
ble at  mid-day  to  the  naked  eye,  and  in  the  absence  of  the 
Moon  casts  a  perceptible  shadow. 

Being  an  inferior  planet,  Venus  is  never  in  opposition 
to  the  sun,  and  is  always  below  the  horizon  at  midnight. 
During  part  of  the  year,  it  rises  before  the  Sun,  and  ushers 
in,  as  it  were,  the  day ;  when  appearing  at  this  time,  the 
ancients  styled  it  Phosphor  or  Lucifer  (the  light-bearer),  and 
we  call  it  the  Morning  Star.  During  the  rest  of  the  year, 
it  rises  after  the  Sun ;  it  was  then  styled  Hesperus  or  Ves- 
per by  the  ancients,  and  is  distinguished  by  us  as  the  Even- 
ing Star.  . 

Venus  is  very  nearly  of  the  same  size  as  the  Earth.  Its  diameter  has 
generally  been  set  down  at  7,900  miles,  somewhat  less  than  the  Earth's.  In 
Herschel's  latest  Tables,  however,  it  is  given  as  8,108,  which  makes  it  a  lit- 
tle larger. 

Venus's  heat  and  light  are  twice  as  great  as  ours.  So  intense  is  its 
brightness  that  variations  in  its  surface  (if  indeed  its  surface  is  not  hid  from 
us  by  a  cloudy  atmosphere)  for  the  most  part  escape  detection,  every  portion 
of  the  disk  being  flooded  with  light.  Yet  spots  have  occasionally  been  seen 
on  its  surface,  and  mountains  have  been  observed  having  an  estimated  height 
of  from  15  to  20  miles.  Venus's  phases,  when  viewed  through  the  telescope, 
are  similar  to  those  of  Mercury  and  the  Moon ;  but  it  never  appears  ex- 
actly full,  being  invisible  at  the  time  when  this  phase  would  otherwise  bo 
presented. 

967.  THE  EAKTH  (0). — The  third  planet  from  the  Sun 
is  the  Earth,  which  we  inhabit. 

The  form  of  the  Earth  is  that  of  an  oblate  spheroid, — 

What  Is  stated  with  respect  to  Mercury's  seasons  ?  966.  What  is  the  second  planet 
from  the  Sun  ?  How  does  it  look  to  us,  and  why  ?  What  proofs  have  we  of  Venus's 
brightness?  When  is  Venus  called  the  Morning,  and  when  the  Evening  Star  ?  How 
does  the  size  of  Venus  compare  with  that  of  the  Earth  ?  How  do  its  heat  and  light 
compare  with  ours  ?  What  have  been  observed  on  Venus's  surface  ?  What  phases 
does  she  present  ?  96T.  What  is  the  third  planet  from  the  Sun  ?  What  is  the  form 


THE  EARTH.  383 

that  is,  a  sphere  flattened  at  the  poles  like  an  orange.  Its 
equatorial  diameter  is  7925.6  miles,  and  its  polar  diameter 
26^  miles  less.  The  circumference  of  a  sphere  is  a  little 
more  than  three  times  as  great  as  its  diameter;  the  dis- 
tance round  the  earth,  therefore,  is  about  25,000  miles. 

The  Earth  is  so  large  that  its  rotundity  is  not  apparent  to  a  person  stand- 
ing on  its  surface.  We  know  it  to  be  round,  however,  in  several  ways. 
1.  Navigators  have  sailed  round  it.  By  keeping  the  same  general  direction, 
east  or  west  (as  far  as  the  land  would  allow),  they  have  arrived  at  the  place 
of  starting.  2.  The  highest  part  of  a  vessel  approaching  in  the  distance  is 
seen  first,  the  lower  part  being  obscured  by  the  rotundity  of  the  earth's  sur- 
face. If  the  earth  were  a  plain,  we  should  see  the  hull  as  soon  as  the  top- 
mast. 

968.  Motions. — The  Earth  turns  on  its  axis  once  in  24 
hours.    This  is  called  its  Diurnal  Motion.    Constantly  bring- 
ing new  points  of  the  surface  before  the  sun,  and  withdraw- 
ing others  from  its  beams,  this  motion  produces  the  suc- 
cession of  day  and  nigbt. 

The  circumference  of  the  earth  being  25,000  miles,  and  a  complete  revo- 
lution being  made  in  24  hours,  it  follows  that  every  point  on  the  equator  must 
revolve  at  the  rate  of  a  little  over  1,000  miles  an  hour.  As  we  go  towards 
the  poles,  circles  drawn  round  the  earth  parallel  to  the  equator  diminish  in 
length,  and  points  situated  on  them  will  consequently  move  with  less  ve- 
locity. At  the  poles  there  is  no  diurnal  motion  at  all. 

969.  The  Earth  has  also  an  Annual  Motion, — about  the 
Sun.     Its  orbit,  like  that  of  the  other  planets,  is  elliptical, 
but  does  not  deviate  much  from  a  circle.     Its  perihelion  is 
3,000,000  miles  nearer  the  Sun  than  its  aphelion;   conse- 
quently at  the  former  point,  other  things  being  equal,  it 
receives  more  heat  than  at  any  other  part  of  its  orbit. 

The  Earth  reaches  her  perihelion  on  the  1  st  of  January  every  year.  Hence 
our  winter  is  somewhat  milder  tlian  that  of  the  southern  hemisphere ;  while 
the  Sun  at  that  period  of  a  southern  summer  is  perceptibly  hotter  than  the 
summer  sun  at  corresponding  latitudes  in  the  north.  The  heat  in  the  inte- 


of  the  Earth  f  What  is  its  equatorial  diameter  ?  Its  polar  diameter  ?  Its  circum- 
ference ?  Why  do  we  not  see  the  roundness  of  the  Earth  ?  How  do  we  know  it  to 
be  round  ?  968.  What  is  meant  by  the  Earth's  Diurnal  Motion  ?  What  does  it  pro- 
duce ?  What  is  the  velocity  of  the  diurnal  motion  at  the  equator  ?  At  the  poles  ? 
At  intermediate  points  ?  969.  What  is  meant  by  the  Earth's  Annual  Motion  ?  What 
Is  the  shape  of  its  orbit  ?  When  does  the  Earth  receive  the  most  solar  heat,  and 
why?  How  do  the  northern  and  southern  winter  and  summer  compare?  Explain 


384  ASTBONOMY. 

rior  of  Australia  at  the  time  the  Earth  reaches  her  perihelion,  is  said  to  be 
more  intense  than  any  known  even  about  the  equator.  Yet  the  difference  of 
distance  is  so  small  compared  with  the  whole,  as  not  very  materially  to  affect 
the  Earth's  temperature ;  nor  has  it  anything  to  do  with  the  change  of  sea- 
sons, as  we  shall  presently  see. 

970.  The  Earth's  orbit  is  nearly  600,000,000  miles  in 
length ;  and  to  get  round  it  in  365^'  5hw*  48m-  48'-  (which 
is  the  period  of  its  revolution  and  constitutes  our  year),  it 
must  travel  over  68,000  miles  an  hour. 

Though  we  are  constantly  moving  with  this  great  velocity,  we  are  uncon- 
scious of  it.  This  is  because  we  have  never  known  what  it  is  to  be  at  abso- 
lute rest ;  and  again,  the  motion  is  perfectly  easy  and  regular,  there  being  no 
obstructions  in  the  way  to  make  us  sensible  of  it. 

971.  The  Earth  in  Space. — Space  extends  infinitely  on 
all  sides  of  the  Earth,  studded  with  stars  at  different  dis- 
tances.   To  us,  however,  the  stars  appear  equally  distant, 
and  seem  to  lie  on  the  inner  surface  of  a  vast  hollow  sphere, 
at  the  centre  of  which  we  are  placed.     For  purposes  of  defi- 
nition and  description,  it  is  often  convenient  thus  to  allude 
to  the  firmament ;  and  the  expressions  "  celestial  arch ", 
"  concave  surface  of  the  heavens  ",  are  used  for  the  pur- 
pose,— not  to  denote  any  real  objects,  but  the  apparent 
arch  or  concave  surface  that  we  may  conceive  to  be  thrown 
around  us. 

972.  Horizon,  Zenith,  Nadir. — The  Sensible  Horizon  is 
the  line  that  bounds  the  view, — that  is,  where  earth  and 
sky  appear  to  meet.     To  an  observer  on  the  ocean,  or  on 
a  vast  plain  where  there  is  nothing  to  obstruct  the  view, 
this  line  is  always  a  circle.    The  plane  passing  through  the 
sensible  horizon,  and  infinitely  extended  through  space,  is 
called  the  Plane  of  the  Sensible  Horizon. 

The  Rational  Horizon  is  a  plane  passing  through  the 

Earth's  centre,  parallel  to  the  plane  of  the  sensible  horizon. 

At  the  Earth  these  planes  are  separated  by  the  distance  between  the  cen- 

the  cause.  Is  the  Earth's  temperature  materially  affected  by  this  difference  of  dis- 
tance? 970.  With  what  velocity  does  the  Earth  travel  round  its  orbit?  Why  are 
we  not  sensible  of  moving?  971.  What  is  meant  by  the  expressions,  "celestial 
arch",  "concave  surface  of  the  heavens"?  972.  What  is  the  Sensible  Horizon? 
What  is  the  Plane  of  the  Sensible  Horizon  ?  What  is  the  Rational  Horizon  ?  What 


THE  ECLIPTIC.  385 

tre  and  the  surface,  or  4,000  miles ;  but  so  small  is  this  distance  compared 
with  that  at  which  the  stars  are  situated  that  the  two  planes  are  regarded  as 
striking  the  celestial  arch  at  the  same  point.  All  heavenly  bodies  above  the 
rational  horizon  at  any  given  point  are  visible,  and  all  below  it  invisible. 

973.  The  Poles  of  the  Horizon  are  two  points  in  the 
heavens  equally  distant  from  the  circle  that  bounds  the 
view.     One  of  these,  the  point  directly  overhead,  is  called 
the  Ze'-nith;   the  opposite  point,  directly  beneath  us,  is 
called  the  Na'-dir. 

Every  point  on  the  Earth'a  surface  haa  a  horizon,  zenith,  and  nadir  of  its 
own;  and  the  horizon,  zenith,  and  nadir  of  every  point  are  constantly 
changing,  owing  to  the  revolution  of  the  Earth  on  its  axis.  Hence,  at  night, 
new  heavenly  bodies  are  constantly  coming  into  view  in  the  east,  while  others 
are  setting  in  the  west. 

974.  The  Ecliptic. — Seen  from  the  Sun,  the  Earth  would 
appear  to  describe  a  circle  round  that  luminary,  among  the 
fixed  stars  on  the  concave  surface  of  the  heavens.     This 
circle  corresponds  with  the  apparent  path  of  the  sun  as 
seen  from  the  Earth,  and  is  called  the  Ecliptic. 

The  plane  of  the  Earth's  equator,  extended  till  it  meets 
the  concave  surface  of  the  heavens,  forms  what  is  called 
the  Celestial  Equator,  or  the  Equinoctial.  The  ecliptic  and 
the  equinoctial  form  an  angle  of  23°  287,  and  this  angle  is 
called  the  Obliquity  of  the  Ecliptic.  The  axis  of  the 
Earth,  therefore,  instead  of  being  perpendicular  to  the  plane 
of  its  orbit,  is  inclined  to  it  at  an  angle  of  (90°  —  23°  28') 
66°  32'. 

975.  The  ecliptic  cuts  the  equinoctial  at  two  points, 
called  Equinoxes,  because  when  the  sun  appears  at  these 
points  the  days  and  nights  are  equal  all  over  the  world. 

The  equinoxes  are  distinguished  as  Vernaf  and  Autumnal.  The  Vernal 
Equinox  is  that  point  at  which  the  sun  crosses  the  equinoctial  from  south  to 
north,  which  takes  place  in  our  spring.  The  Autumnal  Equinox  is  the  point 

is  the  distance  between  the  two  horizons  at  the  Earth  ?  When  they  strike  the  celes* 
tial  arch  ?  Which  of  the  heavenly  bodies  are  visible  at  any  given  point,  and  which 
invisible?  973.  What  are  the  Poles  of  the  Horizon?  What  is  the  Zenith?  The 
Nadir  ?  What  causes  new  heavenly  bodies  to  keep  coming  into  view  at  night  and 
others  to  set  ?  974.  What  is  the  Ecliptic  ?  What  is  the  Celestial  Equator,  or  Equt. 
noctial?  What  is  the  Obliquity  of  the  Ecliptic?  975.  What  are  the  Equinoxes* 
Why  are  they  so  called?  How  are  they  distinguished  ?  What  is  the  Vernal  Equi- 

17 


386  ASTRONOMY. 

at  which  the  sun  crosses  the  equinoctial  from  north  to  south,— and  this  ho 
does  in  our  autumn. 

976.  The  Zodiac. — The  Zodiac  is  a  belt  on  the  concave 
surface  of  the  heavens,  sixteen  degrees  in  width,  eight  of 
which  lie  on  each  side  of  the  ecliptic.  It  is  divided  into 
twelve  Signs,  of  30  degrees  each.  The  zodiac  is  peculiarly 
interesting  to  us,  because^it  is  the  region  within  which  the 
apparent  motions  of  the  Sun,  the  Moon,  and  all  the  greater 
planets,  are  performed. 

The  zodiac  is  so  called  from  a  Greek  word  signifying  animal,  because  its 
signs  were  for  the  most  part  named  after  animals,  of  which  the  stars  in  each 
seemed  to  the  ancients  to  be  so  grouped  as  to  form  rude  outlines.  Such 
groups  of  stars,  which  seem  to  be  situated  near  each  other  because  lying  iu 
the  same  direction  from  us,  are  called  Constellations.  Owing  to  what  is 
known  as  the  Precession  of  the  Equinoxes, — that  is,  the  sun's  completing  its 
revolution  on  the  ecliptic  every  year  before  it  reaches  the  same  point  of  the 
heavens  relatively  to  the  fixed  stars, — the  signs  of  the  zodiac  do  not  now 
correspond  in  position  with  the  constellations  from  which  they  were  named. 
With  the  equinoxes,  on  which  their  position  depends,  they  have  retrograded 
80  degrees  towards  the  west.  The  signs  of  the  zodiac  and  the  constellations 
of  the  zodiac  must  therefore  be  distinguished  from  each  other. 

977.  The  names  of  the  signs  of  the  zodiac  are  given  below  in  Latin  and 
English,  with  the  characters  by  which  they  are  respectively  denoted.  They 
are  given  in  their  order,  commencing  at  the  vernal  equinox. 


f  Aries,  the  ram. 
8  Taurus,  the  bull, 
n  Gemini,  the  twins. 
23  Cancer ',  the  crab. 
SI  Leo,  the  lion. 
TIE.  Virgo,  the  virgin. 


£±  Libra,  the  balance. 

TH,  Scorpio,         the  scorpion. 
#    Sagittarius,  the  archer. 
\3   Capricornus,  the  goat. 
£?  Aquarius,      the  water-bearer. 
X  Pisces,  the  fishes. 


978.  The  Change  of  Seasons. — It  has  been  stated  that 
the  Earth  is  nearer  the  Sun  at  one  period  of  its  revolution 
than  at  another.  The  change  of  seasons,  however,  is  en- 
tirely independent  of  this  fact,  and  is  produced  by  the  sun's 
rays  falling  on  a  given  point  of  the  Earth's  surface  with 
different  degrees  of  obliquity  at  different  parts  of  its  orbit. 

nox?  What  is  the  Autumnal  Equinox ?  976.  What  is  the  Zodiac?  How  is  it  di- 
vided? What  makes  it  peculiarly  interesting  to  us?  From  what  is  the  zodiac  so 
called  ?  What  are  Constellations  ?  How  are  the  signs  of  the  zodiac  now  situated 
relatively  to  the  constellations  from  which  they  were  named  ?  To  what  is  this  ow- 
ing ?  977.  Name  the  signs  of  the  zodiac.  978.  By  what  is  the  change  of  seasons  pro- 


THE   CHANGE    OF   SEASONS. 


387 


When  the  Sun  is  vertical,  or  directly  overhead,  its  heat  is 
most  intense ;  and  the  less  its  rays  deviate  from  a  vertical 
line  in  striking  the  surface,  the  more  heat  they  impart  to  it. 
The  angle  at  which  the  Sun's  rays  strike  a  given  part 
of  the  Earth's  surface  keeps  constantly  varying,  in  conse- 
quence of  the  Earth's  revolving  with  its  axis  always  point- 
ing in  the  same  direction,  or,  as  it  is  generally  expressed, 
everywhere  parallel  to  itself.  This  will  be  understood  from 
Fig.  334. 

In  Fig.  334  the  Fig.  334. 

Earth  is  repre- 
sented as  moving 
round  the  Sun, 
which  is  in  one  of 
the  foci  of  her  el- 
liptical orbit.  The 
dotted  line  is  the 
zodiac,  divided  in- 
to its  twelve  signs. 
N  S  is  the  Earth's 
axis,  which  main- 
tains the  same  di- 
rection in  the  four 
positions  shown, 
and  at  every  other 
part  of  the  orbit. 

At  the  vernal  equinox  (March  21),  the  equator  is  directly  opposite  the 
Sun ;  the  solar  rays  fall  at  the  same  angle  on  the  northern  hemisphere  as  on 
the  southern,  and  it  is  spring  in  the  former,  autumn  in  the  latter.  The 
Earth's  axis  is  inclined  neither  to  nor  from  the  sun ;  consequently,  half  the 
surface,  from  pole  to  pole,  is  enlightened  at  a  time,  and  day  and  night  are  of 
equal  length  all  over  the  globe. 

As  the  Earth  moves  eastward,  the  rays  of  the  Sun  no  longer  fall  verti- 
cally on  the  equator,  but  on  places  north  of  it.  This  continues  till  June  21st, 
when  the  sun  is  vertical  to  places  23°  28'  north  (this  being  the  obliquity  of 
the  ecliptic),  and  his  rays  extend  over  the  same  distance  beyond  the  north 
pole.  It  is  now  summer  in  the  north  and  winter  in  the  south,  for  in  propor- 
tion as  the  solar  rays  fall  less  obliquely  on  the  former,  they  must  fall  more 
obliquely  on  the  latter.  It  will  be  observed,  also,  that  a  space  extending 
23°  28'  around  the  south  pole  is  totally  dark. 


An  tt        ,, 
Equal  Da;j  4  R 


duced  ?  When  is  the  Sun's  heat  most  intense  ?  Why  does  the  angle  at  which  the 
Sun's  rays  strike  a  given  part  of  the  Earth's  surface  keep  varying  ?  What  does  Fig. 
834  represent?  Describe  the  position  of  the  Earth  and  the  circumstances  attending 


388  ASTRONOMY. 

The  Sun  is  never  directly  overhead  to  any  place  farther  north  of  th« 
equator  than  23°  28'.  As  th .  Earth  continues  her  course  eastward,  it  be- 
comes vertical  to  places  more  and  more  to  the  south,  and  by  the  22d  of  Sep- 
tember, or  thereabouts,  it  is  vertical  to  the  equator  just  as  it  was  six  months 
before.  This  is  the  period  of  the  autumnal  equinox.  The  Earth  again  pre- 
sents a  full  side  from  pole  to  pole  to  the  Sun,  and  the  days  and  nights  are 
once  more  equal.  We  have  now  the  southern  spring  and  the  northern 
autumn. 

From  this  point,  the  solar  rays  become  more  and  more  oblique  in  the 
north  and  fall  vertically  on  places  farther  and  farther  south,  till  the  same 
limit  of  23°  28'  is  attained,  which  takes  place  about  December  21,  and  marks 
the  northern  winter  and  the  southern  summer.  Beyond  this  limit  the  Sun  is 
never  directly  overhead.  As  the  Earth  keeps  on  to  the  east,  his  vertical 
rays  fall  on  latitudes  nearer  and  nearer  to  the  equator,  till  finally  on  the  21st 
of  March  places  on  the  equator  have  the  Sun  in  their  zenith  as  they  had  six 
and  twelve  months  before. 

979.  The  explanation  just  given  shows  that  there  are 
two  points  of  the  ecliptic  in  which  the  Sun  is  about  23£  de- 
grees from  the  equator,  and  from  which  he  seems  to  turn 
back  towards  that  line.    These  points  are  called  Solstices 
(standing-points  of  the  Sun),  because  the  Sun  appears  to 
stand  still  for  several  days  at  the  same  place  in  the  heav- 
ens before  taking   an   opposite    direction.      The   solstice 
reached  in  June  is  called  the  Summer  Solstice  ;  that  in  De- 
cember, the  Winter  Solstice. 

980.  Circles  on  the  Earth's  surface  about  23£  degrees 
north  and  south  of  the  equator  form  the  limits  beyond 
which  the  Sun's  rays  are  never  vertical.    These  circles  are 
called  Tropics  (from  a  Greek  word  meaning  to  turn),  be- 
cause on  reaching  them  the  vertical  rays  turn  back  towards 
the  equator.    The  northern  tropic  is  called  the  Tropic  of 
Cancer,  because  when  the  Sun  reaches  this  line  he  is  seen 
from  the  Earth  in  the  sign  Cancer,  as  will  be  apparent  from 
Fig.  334.    For  a  similar  reason  the  southern  tropic  is  called 
the  Tropic  of  Capricorn. 

981.  It  appears  from  Fig.  334  that  from  March  21  to  September  22  the 
north  pole  is  constantly  illuminated  and  the  south  pole  in  darkness,  notwith- 
standing the  revolution  of  the  Earth  on  its  axis ;  while  from  September  22 

it,  at  March  21.  At  June  21.  At  September  22.  At  December  21.  9T9.  What  are 
the  Solstices?  Why  are  they  so  called ?  How  are  they  distinguished ?  980.  What 
«v  tiie  Tropics?  Whence  is  their  name  derived?  What  is  the  northern  tropic 


TIIE  MOON.  389 

to  March  21,  darkness  reigns  at  the  north  pole  and  the  south  pole  enjoys 
continual  light.  At  the  summer  solstice  there  is  a  space  of  231/*  degrees 
about  the  north  pole  on  which  the  Sun  does  not  set,  and  at  the  winter  sol- 
stice a  corresponding  space  about  the  south  pole.  The  lines  that  bound 
these  regions  are  called  the  Polar  Circles.  The  one  near  the  north  pole  is 
called  the  Arctic  Circle ;  that  near  the  south  pole,  the  Antarctic  Circle. 

982.  If,  instead  of  being  inclined,  the  Earth's  axis  were  perpendicular  to 
the  plane  of  its  orbit,  the  regions  on  the  equator  would  have  the  Sun  con- 
stantly in  their  zenith,  day  and  night  would  always  be  equal  over  the  whole 
globe,  there  would  be  no  variety  of  seasons,  and  a  given  place  would  have 
about  the  same  temperature  from  one  year's  end  to  another.  Something  of 
this  kind  must  be  the  case  on  the  planet  Jupiter,  whose  axis  is  nearly  per- 
pendicular to  the  plane  of  its  orbit.  On  the  other  hand,  the  more  the  axis 
of  a  planet  is  inclined,  the  greater  are  the  extremes  of  temperature  incident 
to  its  several  seasons. 

983.  THE  MOON  (•). — The  Earth  is  attended  by  one 
satellite  called  the  Moon, — a  beautiful  orb  which  *  rules  the 
night '  with  its  gentle  brilliancy,  produces  in  part  the  tides, 
and  sensibly  affects  the  Earth's  motions  by  its  attraction. 

984.  Size. — The  Moon's  diameter  is  2,165  miles,  but  its 
apparent  size  is  almost  equal  to  the  Sun's  in  consequence  of 
its  nearness  to  our  planet.     Its  density  is  not  much  more 
than  one-half  that  of  the  Earth,  and  it  contains  about  one- 
eightieth  as  much  matter. 

985.  Motions. — The  Moon  is  240,000  miles  from  the 
Earth,  and  revolves  about  the  latter  so  as  to  reach  the  same 
point  relatively  to  the  fixed  stars  in  27  days,  8  hours.     To 
reach  the  same  point  relatively  to  the  Sun  requires  29  days, 
13  hours,  since  the  Earth  has  itself  meanwhile  advanced  in 
its  orbit. — When  nearest  the  Earth,  the  Moon  is  said  to 
be  in  her  Per'-i-gee,  and  when  farthest  from  it  in  her  Ap'- 
o-gee. 

The  terms  perigee  and  apogee  (which  mean  near  the  Earth  and  away  from 
the  Earth)  are  also  applied  to  the  apparent  position  of  the  Sun.  When  the 
Earth  is  at  its  perihelion,  the  Sun  is  said  to  be  in  perigee ;  and  when  the 
Earth  is  at  its  aphelion,  the  Sun  is  in  apogee. 

called,  and  why?  The  southern?  981.  What  are  the  Polar  Circles?  What  is  the 
one  near  the  north  pole  called  ?  That  near  the  south  pole  ?  982.  If  the  Earth's  axis 
were  perpendicular  to  the  plane  of  its  orbit,  what  would  follow  ?  What  is  said  of 
Jupiter  ?  983.  By  what  is  the  Earth  attended  ?  984  How  great  is  the  Moon's  diam- 
eter? Its  density?  Its  mass?  985.  How  far  is  the  Moon  from  the  Earth ?  What 
is  the  period  of  her  revolution  ?  When  Is  the  Moon  said  to  be  in  perigee  f  In  ap- 


390  ASTRONOMY. 

The  Moon  also  turns  on  its  axis  in  exactly  the  same  time 
that  it  takes  to  revolve  round  the  Earth,  and  in  the  same 
direction.  The  consequence  is  that  she  always  presents  the 
same  side  to  the  Earth.  Nearly  one-half  of  our  fair  at- 
tendant we  never  see,  and  to  the  inhabitants  of  half  her 
surface,  if  she  has  any,  we  are  invisible. 

986.  Phases. — The  Moon  is  non-luminous,  and  shines 
only  by  the  reflected  light  of  the  Sun;  hence  the  hemi- 
sphere presented  to  the  Sun  is  bright,  while  the  opposite 
one  is  dark.  As  the  Sun,  Moon,  and  Earth  are  constantly 
taking  different  positions  relatively  to  each  other,  the  por- 
tion of  illuminated  lunar  surface  presented  to  us  is  as  con- 
stantly changing.  Hence  arise  what  are  called  the  Phases 
of  the  Moon. 

When  new,  the  Moon  lies  between  the  Earth  and  the  Sun,  near  a  line  con- 
necting their  centres.  Her  dark  side  is  then  towards  us,  and  she  is  invisible. 
Soon,  however,  she  gets  so  far  east  of  the  Sun  as  to  appear  in  the  west 
shortly  after  his  setting.  A  bright  crescent  then  becomes  visible  on  the  side 
nearest  the  Sun,  the  rest  of  her  circular  disk  being  just  discernible,  not  by 
sun-light  directly  received,  but  by  sun-light  reflected  from  the  Earth  to  the 
Moon,  and  by  her  reflected  back  to  us.  The  crescent  gradually  grows  larger, 
until,  when  the  Moon  is  90  degrees  from  the  Sun,  or  in  quadrature,  half  her 
disk  is  illumined.  She  is  then  said  to  be  in  her  First  Quarter. 

Each  succeeding  night  now  finds  the  enlightened  portion  larger  and 
larger,  and  the  Moon  is  said  to  be  gibbous.  At  last  she  reaches  a  point  at 
which  she  is  again  almost  in  a  line  with  the  Sun  and  the  Earth,  but  this  time 
the  Earth  is  in  the  middle.  The  Moon  rises  in  the  east  as  the  Sun  sets  in  the 
west ;  the  whole  of  her  enlightened  hemisphere  is  therefore  turned  towards 
us,  and  she  is  said  to  be  full. 

After  this  the  Moon  again  becomes  gibbous,  and  we  see  less  and  less  of 
her  enlightened  surface,  till  at  length  half  of  her  disk  is  dark,  when  she  is 
said  to  be  in  her  Third  Quarter.  Advancing  beyond  her  third  quarter,  she 
wanes  still  further  to  a  crescent,  and  at  length  on  arriving  in  conjunction 
with  the  Sun  disappears  entirely, — to  go  through  the  same  phases  again  as 
she  makes  another  revolution  in  her  orbit. 

987.  To  the  inhabitants  of  the  Moon,  if  any  there  be,  the  Earth  presents 
the  same  phases  that  the  Moon  does  to  us,  but  in  reversed  order.  When  the 
Moon  is  new  to  us,  the  Earth  is  full  to  them, — a  splendid  orb,  thirteen  times 


ogee  ?  When  is  the  Sun  said  to  be  in  perigee  ?  In  apogee  ?  How  long  is  the  Moon 
in  turning  on  her  axis  ?  What  is  the  consequence  ?  986.  What  is  said  of  the  Moon's 
light?  What  causes  her  to  present  different  phases  to  the  Earth?  Describe  the 
phases  successively  presented.  987.  What  phases  does  the  Earth  present  to  the 


THE  MOOX.  391 

as  large  as  the  full  Moon.  When  she  is  in  her  first  quarter,  the  Earth  is  in 
her  third  quarter,  Ac. 

988.  The  Moon  has  either  no  atmosphere  at  all,  or  one 
exceedingly  rare,  and  not  extending  more  than  a  mile  from 
its  surface.  Hence  it  must  be  destitute  of  water,  for  any 
liquid  on  its  surface  would  long  since  have  been  dissipated 
by  the  heat  of  the  lunar  days,  there  being  no  atmospheric 
pressure  to  check  evaporation.  If  there  were  any  water 
on  the  surface  of  the  Moon,  clouds  would  certainly  be  ob- 
served at  times  dimming  its  face. 

989.  Viewed  through  a  telescope,  the  surface  of  the  Moon  appears  ex- 
ceedingly rough,  covered  with -isolated  rocks,  deep  valleys,  yawning  chasms, 
craters  of  extinct  volcanoes,  in  some  cases  more  than  100  miles  in  width,  and 
lofty  mountains,  several  of  which  are  from  three  to  four  miles  high  and  cast 
their  shadows  a  great  distance  over  the  rugged  plains.  Every  thing  is  deso- 
late in  the  extreme.  Several  of  the  earlier  astronomers  thought  that  they 
discerned  volcanoes  in  a  state  of  eruption;  but  later  observers  are  of  the  con- 
trary opinion,  attributing  the  peculiar  brightness  of  the  supposed  volcanic 
summits  to  phosphorescence,  or  superior  reflective  properties. 

Names  have  been  given  to  the  various  mountains  and  spots  visible  on  the 
Moon,  and  a  map  has  been  prepared  of  the  whole  side  presented  to  us,  which 
has  been  pronounced  "  vastly  more  accurate  than  any  map  of  the  Earth  we 
can  yet  produce." — The  great  telescope  of  the  Earl  of  Rosse  shows  with  dis- 
tinctness every  object  on  the  lunar  surface  that  is  100  feet  in  height.  It  has 
brought  to  light,  however,  no  signs  of  life  or  habitation. 

990.  MARS  (  £  ). — Mars,  the  fourth  planet  from  the  Sun, 
is  4,546  miles  in  diameter.  Its  day  is  of  nearly  the  same 
length  as  ours,  its  year  about  twice  as  long.  The  incli- 
nation of  its  axis  to  the  plane  of  its  orbit  does  not  differ 
much  from  th,e  Earth's,  and  its  seasons  are  therefore  simi- 
lar to  ours.  It  is  surrounded  by  an  atmosphere  of  mod- 
erate density. 

Mars  is  easily  distinguished  in  the  heavens  by  his  red  fiery  light,  which 
is  supposed  to  owe  its  color  to  the  soil  from  which  it  is  reflected.  The  tele- 
scope distinctly  shows  continents  of  a  dull  red  tinge,  like  that  of  sand-stone, 

Moon  ?  988.  What  is  said  of  the  Moon's  atmosphere  ?  Why  is  the  Moon  sup- 
posed to  be  destitute  of  water  ?  989.  How  does  the  Moon  look,  when  viewed  through 
a  telescope?  What  is  now  thought  respecting  the  supposed  volcanic  eruptions  for- 
merly observed  ?  How  high  objects  does  the  Earl  of  Eosse's  telescope  distinctly 
show  ?  990.  Which  is  the  fourth  planet  from  the  Sun  ?  What  is  the  length  of  its 
diameter?  Its  day  ?  Its  year  ?  How  do  its  seasons  compare  with  ours  ?  How  may 
Mars  be  distinguished  ?  What  docs  the  telescope  show  ?  What  are  seen  about  the 


392  THE   ASTEROIDS 

washed  by  seas  of  a  greenish  hue.  Bright  white  spots  are  seen  about  the 
poles,  which  are  no  doubt  occasioned  by  the  reflection  of  the  sun's  light  from 
the  snow  and  ice  collected  there.  It  is  observed  that  as  each  pole  is  turned 
towards  the  sun  the  spots  about  it  diminish  in  size,  owing  to  the  melting  of 
the  snow  by  the  solar  heat. 

991.  THE  ASTEROIDS. — The  Asteroids  are  so  small  that, 
with  the  exception  of  one  or  two  which  have  been  seen 
without  a  telescope,  they  are  invisible  to  the  naked  eye. 
Their  diameters  have  not  yet  been  accurately  determined ; 
some  exceed  1 00  miles,  and  others  probably  fell  somewhat 
under  that  mark.    A  number  of  them  are  provided  with 
extensive  atmospheres.      The  Asteroids  are  supposed  by 
some  to  be  the  wreck  of  one  large  planet,  which  they  be- 
lieve to  have  originally  revolved  between  Mars  and  Jupi- 
ter, and  by  some  tremendous  catastrophe  to  have  burst 
into  fragments.     Many  similar  bodies  probably  remain  to 
be  discovered  in  this  region. 

The  Asteroids  are  comparatively  so  diminutive  that  the  force  of  gravity 
on  their  surfaces  must  be  very  small.  A  man  placed  on  one  of  them  would 
spring  with  ease  60  feet  high,  and  sustain  no  greater  shock  in  his  desceot 
than  he  does  on  the  earth  from  leaping  a  yard.  On  such  planets  giants  may 
exist ;  and  those  enormous  animals  which  here  require  the  buoyant  power 
of  water  to  counteract  their  weight,  may  there  inhabit  the  land. 

992.  JUPITER  (H). — Next  to  the  asteroids  is  Jupiter, 
the  largest  of  the  planets,  which  exceeds  the  Earth  in  bulk 
nearly  1,300  times.     Its  revolution  round  the  Sun  is  per- 
formed in  about  12  years,  and  that  around  its  axis  in  less 
than  10  hours.  Jupiter  is  attended  by  four  satellites,  which 
revolve  about  it  from  west  to  east. 

All  of  these  satellites  but  one  exceed  our  Moon  in  size.  The  largest  would 
sometimes  be  visible  to  the  naked  eye  as  a  very  faint  star,  were  it  not  lost  in 
the  superior  brightness  of  its  planet.  Three  of  them  are  totally  eclipsed 
during  every  revolution  by  the  long  shadow  which  the  planet  casts,  and  the 
fourth  is  very  often  eclipsed.  The  relation  between  their  orbits  and  motions 
is  such  that  for  many  years  to  come  Jupiter  will  never  be  deprived  of  the 
light  of  all  four  at  the  same  time. 

poles?  By^what  are  they  supposed  to  be  caused?  991.  Are  the  Asteroids  visible  to 
the  naked  eye  ?  What  is  the  length  of  their  diameters  ?  What  are  the  Asteroids 
thought  by  many  to  be  ?  What  is  stated  with  respect  to  the  force  of  gravity  on  their 
surface  ?  992.  How  does  Jupiter  rank  in  size  ?  How  does  it  compare  in  bulk  with 
toe  Earth?  What  is  the  length  of  its  year?  Its  day?  By  what  is  it  attended  ? 


SATTJKN.  393 

So  large  is  Jupiter,  and  so  short  a  time  is  it  in  revolving  on  its  axis,  that 
every  point  on  its  equator  must  turn  at  the  rate  of  450  miles  a  minute.  The 
result  is  an  immense  excess  of  centrifugal  force  at  the  equator ;  and  this  is 
seen  to  have  operated  before  the  mass  of  the  planet  became  hard,  by  flatten- 
ing it  very  much  at  the  poles. — Jupiter's  disk  is  always  crossed  with  a  num- 
ber of  dark  parallel  belts,  as  shown  in  Fig.  331.  They  vary  in  breadth  and 
situation,  but  are  always  parallel  to  the  equator  of  the  planet ;  hence  they 
appear  to  be  connected  with  its  rotation  on  its  axis,  and  are  no  doubt  pro- 
duced by  disturbances  in  its  atmosphere. 

992.  SATURN  ( b ). — Saturn,  which  is  next  to  Jupiter  in 
distance  from  the  Sun,  is  also  next  to  it  in  size,  having  a 
diameter  of  76,791  miles,  and  consequently  a  bulk  nearly 
1,000  times  that  of  the  Earth.     Its  day  is  not  half  so  long 
as  ours  ;  but  it  is  294-  of  our  years  in  making  one  complete 
revolution  in  its  orbit. 

Saturn  has  eight  moons,  seven  of  which  were  known  for  sixty  years  be- 
fore the  eighth  was  discovered.  The  largest  of  them  has  a  diameter  about 
half  as  large  again  as  our  Moon.  Saturn's  disk,  like  Jupiter's,  is  frequently 
diversified  with  belts ;  spots  are  of  rare  occurrence.  An  atmosphere  of  con- 
siderable density  is  supposed  to  surround  the  planet. 

Saturn  has  a  remarkable  appendage,  consisting  of  three  bright,  flat,  and 
exceedingly  thin  rings,  encircling  its  equator,  and  revolving  with  it  around 
its  axis  in  about  the  same  time  in  which  the  planet  itself  revolves.  The 
whole  breadth  of  these  rings  is  27,000  miles,  while  their  thickness  does  not 
exceed  100  miles.  They  are  supposed  to  consist  of  a  mixture  of  gases  and 
vapors,  sufficiently  substantial  to  cast  a  shadow.  The  three  rings  are  de- 
tached from  each  other,  and  lie  in  the  same  plane  very  close  together,  while 
the  inner  one  is  19,000  miles  from  the  surface  of  the  planet.  They  are  pre- 
vented from  falling  in  upon  the  planet  by  the  centrifugal  force  generated  by 
their  rapid  revolution. 

993.  UKANUS  ( *J* ). — Uranus,  the  next  planet  to  Saturn, 
revolves  about  the  Sun  in  84  of  our  years.     There  being  no 
spots  on  its  surface,  we  are  unable  to  fix  the  period  of  its 
revolution  on  its  axis.     It  is  attended  by  six  moons,  which 
move  from  east  to  west  (unlike  the  satellites  of  the  other 

"What  is  the  size  of  the  largest  of  these  moons  ?  What  relation  subsists  between  their 
orbits  and  motions  ?  What  is  the  shape  of  Jupiter  ?  What  has  caused  the  flattening 
nt  the  poles  ?  With  what  is  Jupiter's  disk  crossed  ?  To  what  are  these  belts  to  be 
attributed?  992.  What  is  the  next  planet  to  Jupiter?  What  is  Saturn's  diameter? 
How  does  its  bulk  compare  with  the  Earth's?  Its  day?  Its  year?  How  many 
moons  has  Saturn ?  How  is  its  disk  diversified?  What  remarkable  appendage  has 
Saturn?  Describe  its  rings.  993.  What  is  the  next  planet  to  Saturp  ?  What  is  the 
length  of  the  year  of  Uranus  ?  Its  day  ?  By  what  is  it  attended  ?  How  do  its  light 

17* 


394  ASTRONOMY. 

planets)  in  orbits  nearly  perpendicular  to  that  of  the  planet. 
The  solar  heat  and  light  of  Uranus  are  only  ^^7  of  ours. 

994.  NEPTUNE  ( ¥  ). — Neptune,  the  most  remote  planet 
of  the  solar  system,  is  invisible  to  the  naked  eye.     Seen 
through  the  telescope,  it  looks  like  a  star  of  the  eighth 
magnitude.      The  diameter  of  Neptune  is  39,800  miles, 
which    is   4,500    more    than   that   of    Uranus.    Its  rev- 
olution around  the  Sun  is  performed  in  about  165  of  our 
years.   Neptune  has  at  least  one  moon,  distant  from  it  about 
as  far  as  ours  is  from  us. 

The  discovery  of  Neptune  is  one  of  the  greatest  triumphs  of  which  sci- 
ence can  boast.  Comparing  observations  on  Uranus,  while  it  was  still 
thought  to  be  the  most  distant  member  of  the  solar  system,  astronomers 
found  certain  perturbations  or  irregularities,  in  its  motions,  which  could  be 
accounted  for  only  on  the  supposition  that  there  was  some  unknown  planet 
beyond  it  by  whose  attraction  it  was  affected.  Le  Verrier  thoroughly  inves- 
tigated the  subject,  and  even  went  so  far  as  to  compute  the  size  and  distance 
of  the  suspected  planet,  and  to  predict  in  what  part  of  the  heavens  it  would 
be  found  at  a  given  date.  A  letter  from  the  French  astronomer,  embracing 
the  results  of  his  calculations,  reached  Berlin,  September  13, 1846 ;  and  that 
very  evening,  sweeping  the  heavens  with  his  powerful  telescope,  according 
to  Le  Terrier's  instructions,  Dr.  Galle  discovered  what  was  apparently  a  star 
of  the  eighth  magnitude  not  laid  down  on  his  chart,  but  was  proved  by  its 
change  of  place  on  the  following  evening  to  be  a  planet. — It  is  just  to  add 
that  Adams,  an  English  astronomer,  had,  about  the  same  time  with  Le  Ver- 
rier, made  similar  calculations,  and  with  nearly  the  same  result. 

995.  REAL  AND  APPARENT  POSITION  OF  THE  HEAVENLY 
BODIES. — We  seldom  see  the  heavenly  bodies  in  their  real 
position.     This  is  owing  to  two  causes, — Refraction  and 
Parallax. 

996.  Effect  of  Refraction. — Refraction,  which  has  been 
explained  in  the  chapter  on  Optics,  bends  rays  of  light  en- 
tering our  atmosphere  from  a  rarer  medium,  and  causes  the 
body  from  which  they  proceed  to  appear  higher  than  it 
really  is.     The  Sun  is  thus  made  visible  a  few  moments  be- 
fore he  actually  rises  and  after  he  sets.    The  effect  of  re- 

and  heat  compare  with  ours?  991  "What  is  the  most  remote  planet  of  the  solar  sys- 
tem ?  How  does  Neptune  look,  when  seen  through  the  telescope  ?  What  is  its  diam- 
eter ?  What  is  the  period  of  its  revolution  ?  How  many  moons  has  Neptune  ?  Give 
an  account  of  the  circumstances  under  which  Neptune  was  discovered.  995.  Why  do 
we  not  see  the  heavenly  bodies  in  their  real  position  ?  996.  What  is  the  effect  of  re- 


PARALLAX. 

fraction  is  greatest  when  a  body  is  on  the  ! 
diminishes  as  it  ascends  towards  the  zenith,  at  which^ 
it  entirely  disappears.  ^*/h 

997.  Effect  of  Parallax. — A  planet  seen  from  different^ 
points  of  the  Earth's  surface  appears  to  lie  in  different 
positions.    This  is  evident  from  Fig.  335. 

The  planet  C  to  an  observer                               _,.    ^ 
at  A  seems  to  lie  at  F ;  to  one           ., 
at  B  it  appears  to  lie  at  D.    To     s — ^--^___^           „ 
avoid  the  inconsistencies  which    (    E    L  "     ~      •*••—• ' "*^ 


would  otherwise  exist  in  obser-        ^  «j» 

vations  made  at  different  places, 

the  centre  of  the  earth  is  taken  as  a  standard  point ;  and  the  true  position 
of  a  heavenly  body  is  that  point  of  the  celestial  arch  which  would  be  cut 
by  a  line  connecting  the  centre  of  the  Earth  with  the  centre  of  the  body  in 
question,  infinitely  produced. 

Parallax  is  the  angle  made  by  a  line  from  a  heavenly 
body  to  the  Earth's  centre  and  another  line  from  the  same 
body  to  the  eye  of  an  observer. 

It  is  evident  that,  the  nearer  a  heavenly  body  is,  the  greater  is  its  parallax 
The  fixed  stars  are  so  remote  that  they  have  no  appreciable  parallax.  The 
Earth,  if  visible  to  them,  would  be  nothing  more  than  a  minute  point  of  light. 
— The  parallax  of  a  heavenly  body  is  greatest  when  it  is  on  the  horizon.  At 
the  zenith  it  would  be  nothing,  because  from  that  point  the  lines  to  the  ob- 
server's eye  and  the  centre  of  the  Earth  would  coincide. 

998.  ECLIPSES. — By  an  Eclipse  of  the  Sun  or  Moon  is 
meant  its  temporary  obscuration  by  the  interposition  of 
some  other  body.    An  eclipse  is  called  Total,  when  the 
whole  disk  is  obscured ;  and  Partial,  when  only  a  portion 
is  darkened. 

999.  An  eclipse  of  the  Sun  is  caused  by  the  Moon's  get- 
ting between  it  and  the  Earth,  and  intercepting  its  rays. 
This  can  happen  only  at  new  Moon,  because,  when  between 
us  and  the  Sun,  the  Moon  must  present  to  as  her  unenlight- 
ened side. 

fraction  ?  99T.  How  does  a  planet  seem  to  lie,  when  observed  from  different  parts  of 
the  Earth's  surface  ?  Illustrate  this  with  Fig.  885.  What  is  the  true  position  of  a 
heavenly  body  ?  What  is  Parallax?  "What  is  said  of  the  parallax  of  the  fixed  stars? 
What  would  be  the  effect  of  refraction  and  parallax  on  the  apparent  position  of  a 
body  in  our  zenith  ?  998.  What  is  an  Eclipse  ?  When  is  an  eclipse  called  Total,  and 
when  Partial  ?  999.  What  causes  an  eclipse  of  the  Sun  T  When  alon«  <5an  this  hap- 


396  ASTRONOMY. 

If  the  Moon's  orbit  lay  in  the  same  plane  as  the  Earth's,  she  would  eclipse 
the  Sun  every  time  she  became  new ;  but,  as  her  orbit  is  inclined  to  the 
ecliptic  at  an  angle  of  more  than  5  degrees,  her  shadow  may  fall  above  or 
below  the  Earth  at  the  time  of  her  change. 

When  the  Moon  intervenes  between  the  Sun  and  the  Earth  at  such  a  dis- 
tance from  the  latter  as  to  make  her  apparent  diameter  less  than  the  Sun's,  a 
singular  phenomenon  is  exhibited.  The  whole  disk  of  the  Sun  is  obscured, 
except  a  narrow  ring  around  the  outside  encircling  the  darkened  centre. 
This  is  called  an  Annular  Eclipse,  from  the  Latin  annulus,  a  ring. 

1000.  An  eclipse  of  the  Moon  is  caused  by  the  Earth's 
getting  between  it  and  the  Sun.  This  can  take  place  only 
at  full  Moon,  because  when  the  Earth  is  between  the  Sun 
and  the  Moon  the  latter  must  present  her  enlightened  side 
to  the  Earth. 

Non-luminous  itself,  when  cut  off  from  the  solar  rays,  the  Moon  must  be- 
come invisible.  There  is  this  difference  between  an  eclipse  of  the  Sun  and 
the  Moon.  In  the  former,  the  Sun  shines  the  same  as  ever,  and  its  bright- 
ness is  undiminished  to  those  who  are  out  of  the  Moon's  shadow.  "When  the 
Moon  is  eclipsed,  on  the  other  hand,  she  diffuses  no  light,  and  is  dark  to  all 
within  whose  range  of  vision  she  is  situated. — Solar  eclipses  occur  more  fre- 
quently than  lunar.  The  greatest  number  of  both  that  can  take  place  in  a 
year,  is  seven ;  the  smallest  number,  two ;  the  usual  number,  four. 

1001.  When  the  Sun  is  totally  eclipsed,  the  heavens  are  shrouded  in  dark- 
ness, the  stars  make  their  appearance,  the  birds  go  to  roost,  the  animals  by 
their  uneasiness  testify  their  alarm,  and  all  nature  seems  to  feel  the  unnatu- 
ral deprivation  of  the  light  of  day.  It  is  not  surprising  that,  when  the  cause 
of  the  phenomenon  was  unknown,  it  filled  the  minds  of  men  with  consterna- 
tion. Even  at  the  present  day  barbarous  nations  regard  eclipses  as  indica- 
tions of  the  displeasure  of  their  gods.  Columbus,  on  one  occasion,  when 
wrecked  on  the  coast  of  Jamaica,  and  in  imminent  danger  both  of  starvation 
and  an  attack  from  the  Indians,  saved  himself  and  his  men  by  taking  advan- 
tage of  this  superstitious  feeling.  From  his  acquaintance  with  astronomy, 
he  knew  that  an  eclipse  of  the  Moon  was  about  to  take  place ;  and  on  the 
morning  of  the  day,  summoning  the  natives  around  him,  he  informed  them 
that  the  Great  Spirit  was  displeased  because  they  had  not  treated  the  Span- 
iards better,  and  would  shroud  his  face  from  them  that  night.  When  the 
Moon  became  dark,  the  Indians,  convinced  of  the  truth  of  his  words,  hastened 
to  him  with  plentiful  supplies,  praying  that  he  would  beseech  the  Great 
Spirit  to  receive  them  again  into  favor. 

pen  ?  Why  is  not  the  Sun  eclipsed  every  time  the  Moon  becomes  new  ?  What  is  an 
Annular  Eclipse  ?  1000.  By  what  is  an  eclipse  of  the  Moon  caused  ?  When  can  this 
take  place  ?  What  difference  is  mentioned  between  an  eclipse  of  the  Sun  and  the 
Moon  ?  Which  occurs  more  frequently  ?  What  is  the  usual  number  in  a  year  ? 
1001.  Describe  the  appearance  of  things  during  a  total  eclipse  of  the  Sun.  How  do 
barbarous  nations  regard  eclipses  ?  How  did  Columbus  once  save  himself  and  his 


COMETS.  397 

1002.  COMETS. —  Comet  is  derived  from  a  Greek  word 
meaning  hair;  and  the  term  is  applied  to  a  singular  class 
of  bodies  belonging  to  the  solar  system,  from  which  long 
trains  of  light,  called  tails,  spread  out  like  hair  streaming 
on  the  wind.    They  differ  very  much  in  appearance ;  but, 
for  the  most  part,  they  consist  of  a  nucleus,  which  is  a  very 
bright  spot,  apparently  denser  than  the  other  portions ;  an 
envelope,  which  is  a  luminous  fog-like  cover  surrounding 
the  nucleus ;  and  a  tail,  which  appears  to  be  an  expansion 
of  the  envelope  produced  by  solar  heat. 

The  tails  of  different  comets  differ  greatly  in  shape  and  extent.  In  some 
this  appendage  is  entirely  wanting ;  in  others  it  has  been  found  to  extend 
120,000,000  miles.  Several  tails  have  been  exhibited  at  the  same  time ;  the 
comet  of  1744  threw  out  no  less  than  six,  like  an  enormous  fan,  over  the 
heavens.  Even  in  the  same  comet  the  tail  keeps  changing,  being  largest 
when  near  the  Sun  and  diminishing  as  it  recedes  from  that  body. — The  tail 
lies  on  the  opposite  side  of  the  nucleus  from  the  Sun, — behind  it,  when  ap- 
proaching its  perihelion,  and  preceding  it  when  retiring  from  that  point. 

1003.  Constitution. — The  matter  of  which  comets  are 
composed  must  be  an  exceedingly  thin  gas  or  vapor. 

The  nucleus  is  always  bright,  no  matter  what  position  in  relation  to  the 
Earth  it  may  occupy ;  no  phases  are  presented,  as  in  the  case  of  the  planets ; 
this  proves  the  nucleus  to  be  so  rare  that  the  solar  light  (which  alone  ren- 
ders it  visible)  can  penetrate  it  and  be  seen  on  the  side  opposite  to  that  which 
it  strikes.  Again,  comets  have  on  different  occasions  passed  very  near  the,, 
planets,  yet  have  never  been  found  to  cause  any  irregularities  in  their  mo- 
tions, while  their  own  motions  have  been  materially  affected.  The  tail,  in 
particular,  must  be  exceedingly  rare,  perhaps  not  weighing  more  than  a  few 
ounces,  even  when  most  extensive. 

1004.  Orbits,  Velocity. — The  orbits  of  the  comets  are 
either  ellipses,  parabolas,  or  hyperbolas. 

If  ellipses,  they  generally  deviate  very  much  from  a  circle,  being  length- 
ened out  an  immense  distance  in  proportion  to  their  breadth.  Comets  that 
move  in  elliptical  orbits  return  after  a  series  of  years ;  those  that  move  in 
parabolas  or  hyperbolas  never  reappear,  but  after  wheeling  about  the  Sun 
dash  off  into  the  remote  regions  of  space,  perhaps  to  visit  other  systems. 

Some  comets  at  their  perihelion  pass  very  close  to  the  Sun.   The  one  that 

men  ?  1002.  "What  are  Comets  ?  Of  what  do  they  consist  ?  "What  is  said  of  the  tails 
of  different  comets?  In  the  case  of  the  same  cornet,  what  change  takes  place  in  the 
tail  ?  How  does  the  tail  lie  ?  1003.  Of,  what  kind  of  matter  must  comets  be  com- 
posed ?  How  is  it  proved  that  the  matter  of  comets  must  be  exceedingly  rare  ? 
1004.  What  shape  are  the  orbits  of  comets  ?  In  what  case  will  the  comet  return  f 


398  ASTRONOMY. 

appeared  in  1843  almost  grazed  its  surface,  approaching  so  near  it  that  the 
solar  disk  must  have  appeared  47,000  times  larger  than  it  looks  to  us,  and 
the  heat  received  must  have  been  twenty -five  times  greater  than  that  re- 
quired to  melt  rock-crystal. 

1005.  When  near  the  Sun,  comets  move  with  incredible 
velocity, — sometimes  at  the  rate  of  over  a  million  miles  an 
hour. 

1006.  Number. — The  exact  number  of  comets  can  not 
be  determined.     Over  seven  hundred  have  been  seen  and 
enumerated.    Multitudes  have  visited  our  system  without 
being  seen  from  the  Earth,  in  consequence  of  reaching  their 
perihelion  in  the  day-time,  or  when  the  heavens  were  ob- 
scured by  mists  and  clouds.    Arago  estimated  the  number 
that  have  appeared  or  will  appear  within  the  orbit  of  Ura- 
nus at  7,000,000 ;  the  same  calculation  extended  to  Nep- 
tune's orbit  would  make  the  number  28,000,000. 

1007.  Comets  were  formerly  regarded  with  superstitious  terror  as  precur- 
sors of  war,  famine,  and  other  misfortunes.  In  more  modern  .times  the  fear 
of  a  collision  made  them  formidable  objects.  This  fear,  however,  has  been 
dispelled  by  the  discovery  of  their  great  rarity.  A  collision,  however  fatal 
it  might  be  to  the  comet,  would  probably  do  little  injury  to  a  solid  body  like 
the  Earth. 

The  Fixed  Stars. 

1008.  The  Fixed  Stars  are  so  called  in  contradistinction 
to  the  planets,  because  they  maintain  the  same  position  rela- 
tively to  each  other,  not  because  they  are  absolutely  at  rest. 
They  all  move  about  some  fixed  point  in  immense  orbits, 
which  it  will  take  millions  of  years  for  them  to  complete. 
Shining  by  their  own  light  and  not  by  reflection,  they  are 
suns,  and  are  probably  each  the  centre  of  a  system  of  its 
own. 

1009.  Magnitudes^ — Varying  in  size  and  situated  at  different  distances 
from  us,  the  stars  are  not  all  of  the  same  brilliancy.  They  are  divided  into 
about  twenty  classes  according  to  their  brightness,  and  distinguished  as  stars 

How  near  did  the  comet  of  1843  pass  to  the  Sun  ?  1005.  What  is  the  velocity  of  com- 
ets, when  near  the  Sun  ?  1006.  What  is  the  number  of  the  comets  ?  What  prevents 
us  from  seeing  many  that  visit  our  system  ?  What  was  Arago's  estimate  ?  1007.  How 
•were  comets  formerly  regarded?  How  are  they  now  looked  upon?  1008.  "Why  are 
the  Fixed  Stars  so  called?  1009.  How  are  the  fixed  stars  classified?  What  are  Tel- 


THE  FIXED   STARS.  399 

of  the  First,  Second,  &c.,  Magnitude;  The  stars  of  the  first  six  magnitudes 
are  visible  to  the  naked  eye ;  the  rest  are  called  Telescopic  Stars,  because 
seen  only  with  the  telescope.  There  are  about  24  stars  of  the  first  magni- 
tude, 50  of  the  second,  and  200  of  the  third ;  but  the  number  in  the  lower 
classes  increases  so  rapidly  as  to  be  almost  bej^ond  enumeration. 

1010.  Constellations. — For  convenience  of  reference,  the  stars  are  divided 
into  constellations,  or  groups,  named  after  animals  and  other  objects  to  which 
their  outline  bears  some  fancied  resemblance.  The  twelve  constellations  of 
the  zodiac  have  been  already  named ;  there  were  thirty-six  more  laid  off  by 
the  ancients  in  other  parts  of  the  heavens.  The  whole  number  has  been  in- 
creased in  modern  times  to  ninety -three.  The  stars  in  each  constellation  are 
distinguished,  according  to  their  magnitude,  first  by  the  letters  of  the  Greek 
alphabet,  then  by  those  of  the  Roman,  and  when  both  are  exhausted,  by  figures. 

1011.  Distance. — The  distance  of  the  fixed  stars  is 
absolutely  incredible.  None  of  them  can  be  less  than 
19,200,000,000,000  miles  from  the  Earth,  while  the  greater 
part  are  far  more  remote. 

The  recent  improvements  in  telescopes  have  enabled  astronomers  to  com- 
pute the  distance  of  nine  of  the  nearest  stars.  Sirius,  the  brightest  of  them, 
is  found  to  be  so  far  off  that  light,  with  a  velocity  of  192,000  miles  a  second, 
is  fourteen  years  in  reaching  us ;  from  the  North  Star  it  is  over  48  years. 
The  mind  is  lost  in  trying  to  comprehend  such  mighty  distances ;  and  yet  it 
will  be  remembered  these  are  among  the  nearest  stars. 

1012.  Several  remarkable  facts  are  worthy  of  note  in  connection  with  the 
fixed  stars.  Some  of  them  wane  for  a  time,  so  as  to  be  classed  in  a  lower 
magnitude,  and  then  resume  their  former  brilliancy.  Others,  after  vanishing 
entirely  for  a  season,  suddenly  reappear ;  these  are  called  Periodical  Stars. 

Many  stars  (at  least  several  thousand),  when  viewed  through  a  powerful 
telescope,  are  resolved  into  two  stars,  one  of  which  is  generally  much  fainter 
than  the  other.  These  are  known  as  Binary  or  Double  Stars.  In  some  cases 
the  faint  one  may  only  appear  to  be  near  the  bright  one  from  lying  in  the 
same  direction,  and  really  be  millions  of  miles  behind  it ;  but  there  is  gen- 
erally reason  for  supposing  that  the  fainter  luminary  revolves  about  the 
brighter  one  in  obedience  to  that  same  great  law  of  gravitation  which  pre- 
vails in  our  own  system. — Some  stars,  apparently  single,  are  resolved  into 
three,  four,  and  even  six,  by  the  telescope.  -  -•  •*&-•-; 

Many  of  the  binary  stars  are  tinged  with  complementary  colors.  The 
larger  one  is  orange-colored,  the  smaller  blue ;  or  the  one  is  red,  and  the 

escopic  Stars  ?  How  many  stars  are  there  of  the  first  magnitude  ?  Of  the  second  ? 
Of  the  third?  1010.  How  are  the  fixed  stars  divided?  How  many  constellations 
were  laid  off  by  the  ancients  ?  How  many  have  been  added  in  modern  times  ?  How 
are  the  stars  in  each  constellation  distinguished  ?  1011.  "What  is  the  distance  of  the 
fixed  stars?  "What  is  the  distance  of  Sirius?  Of  the  North  Star?  1012.  "What  are 
Periodical  Stars  ?  "What  are  Binary  Stars  ?  "What  relation  seems  to  subsist  between 
the  brighter  and  fainter  star  ?  Into  what  are  some  stars  resolved  ?  With  what  are 


400  ASTRONOMY. 

other  green.  Some  of  the  single  stars  look  blood-red  ;  but  there  are  non* 
that  exhibit  deep  tinges  of  blue  or  green. 

The  size  of  several  of  the  fixed  stars  has  been  calculated  approximately. 
Their  diameters  are  found  to  be  enormous, — in  one  case  not  less  than 
200,000,000  miles.  Sirius,  "the  dog-star",  if  set  in  the  place  of  our  Sun, 
would  look  125  times  as  large  as  he,  and  give  us  125  times  as  much  light. 
Trillions  of  miles  away,  as  it  is,  it  dazzles  the  eye  when  seen  through  a  pow- 
erful telescope. 

1013.  THE  GALAXY.— The  Galaxy,  or  Milky  Way,  is  a 
broad  zone  of  light  which  stretches  across  the  sky  from 
horizon  to  horizon,  encircling  the  whole  sphere  and  main- 
taining the  same  position  relatively  to  the  stars.     Exam- 
ined through  a  powerful  telescope,  it  is  found  to  consist 
entirely  of  stars,  scattered  by  millions,  like  glittering  dust, 
on  the  black  ground  of  the  heavens. 

1014.  NEBULAE. — Nebulae  are  clusters  of  stars  so  distant 
that  they  look  like  faint  patches  of  cloud  hardly  discernible 
in  the  sky.     They  vary  in  shape,  and  are  seen  in  different 
quarters  of  the  heavens. 

Lord  Rosse's  great  telescope  resolves  some  of  the  nebulae  into  individual 
stars ;  it  makes  others  appear  bright,  but  not  sufficiently  so  to  be  separated 
into  the  stars  that  compose  them ;  and  it  calls  up  from  the  depths  of  space 
others  which  appear  as  faint  even  to  its  mighty  magnifying  power  as  those 
which  it  resolves  appear  to  the  unaided  eye.  The  milky  way  is  itself  one  of 
these  nebulae,  more  distinct  than  the  others  because  nearer  to  us. 

From  the  facts  set  forth  we  may  conclude  that  the  universe  consists  of  a 
vast  number  of  distinct  clusters  of  worlds,  separated  from  each  other  by  im- 
mense intervals ;  that  the  fixed  stars,  the  milky  way,  our  Sun  and  its  system, 
form  one  of  these  clusters ;  that  the  various  nebulae  constitute  other  clusters, 
fainter  or  brighter  according  to  their  distance  from  us, — each  composed  of 
many  different  systems, — and  having  its  members  separated  as  widely  as  our 
Sun  is  from  the  brother  suns  about  him. 

How  can  the  mind  take  in  such  mighty  thoughts !  How  can  the  heart 
refuse  its  homage  to  the  great  Creator  of  all  these  worlds  I 

many  of  the  binary  stars  tinged?  "What  hai  been  found  with  respect  to  the  diame- 
ters of  some  of  the  fixed  stars?  How  would  Sirins  look,  if  set  in  the  place  of  our 
Sun  ?  1013.  "What  is  the  Galaxy  ?  How  does  it  look  through  a  powerful  telescope  ? 
1014.  What  are  Nebuhe  ?  How  do  nebulae  look  through  Lord  Rossc's  telescope  ? 
What  may  we  conclude  from  the  facts  set  forth  ? 


METEOROLOGY.  401 


CHAPTER  XIX. 

METEOROLOG-Y, 

1015.  METEOROLOGY  is  the  science  which  treats  of  the 
phenomena  of  the  atmosphere.     Among  these  are  winds, 
clouds,  fog,  dew,  rain,  snow,  and  hail. 

Some  of  the  phenomena  of  the  atmosphere  have  been  already  described 
and  explained  in  connection  with  the  various  subjects  that  have  engaged  our 
attention. 

1016.  WIND. — Wind  is  air  put  in  motion. 

The  motion  of  the  air  is  the  result  of  changes  constantly  going  on  in  the 
earth's  temperature,  in  consequence  of  the  alternation  of  day  and  night  and 
the  succession  of  the  seasons.  Those  portions  of  the  atmosphere  that  rest 
on  the  hotter  regions  of  the  earth  become  heated  and  rarefied,  and  rising 
leave  a  vacuum  which  is  immediately  filled  by  a  rush  of  cooler  air  from  the 
surrounding  parts.  Currents  are  thus  produced,  which  we  call  winds. 

The  direction  of  the  wind  is  determined  by  various  local  causes,  modified 
by  the  revolution  of  the  earth  on  its  axis.  The  latter,  operating  alone,  would 
make  it  appear  to  blow  uniformly  from  the  east ;  but  the  various  projections 
on  the  earth's  surface,  and  the  unequal  distribution  of  land  and  water  (the 
latter  of  which  is  incapable  of  being  heated  to  the  same  degree  as  the  former), 
— these  and  other  agencies  constantly  at  work  combine  to  give  the  wind  dif- 
ferent directions  at  different  places,  and  to  make  it  vary  at  the  same  place. 

1017.  Velocity. — The  velocity  of  the  wind  is  measured 
with  an  instrument  called  the  An-e-mom'-e-ter. 

There  are  several  kinds  of  anemometers.  One  of  the 
best  consists  of  a  small  windmill,  with  an  index  attached 
for  recording  the  number  of  revolutions  made  in  a  second. 

It  is  found  with  the  anemometer  that  a  wind  so  slight  as  hardly  to  stir  the 
leaves  travels  at  the  rate  of  1  mile  an  hour ;  a  gentle  wind,  5  miles  in  the 
same  time ;  a  brisk  gale,  15  miles ;  a  high  wind,  30 ;  a  storm,  50 ;  a  hurri- 
cane, 80 ;  a  violent  hurricane,  100. 

1018.  Kinds. — There  are  three  kinds  of  winds;  Con- 
stant, Periodical,  and  Variable. 

1015.  What  is  Meteorology  ?  Mention  some  of  the  phenomena  of  the  atmosphere. 
1016.  What  is  Wind?  What  puts  the  air  in  motion  ?  By  what  is  the  direction  of  the 
wind  determined?  1017.  How  is  the  velocity  of  the  wind  measured ?  What  is  one 
of  the  best  forms  of  the  anemometer  ?  How  fast  does  a  scarcely  perceptible  wind 
travel?  A  gentle  wind?  A  brisk  gale?  A  storm?  A  hurricane?  1018.  How  many 


402  METEOROLOGY. 

1019.  Constant  Winds  are  those  that  blow  throughout 
the  year  in  the  same  direction. 

The  most  noted  of  these  are  the  Trade  Winds,  which  extend  about  30  de- 
grees on  each  side  of  the  equator,  a  zone  of  6  degrees  near  the  centre  known 
as  the  Region  of  Calms  being  excepted.  They  blow  uninterruptedly  on  the 
ocean  from  north-east  to  south-west  in  the  northern  hemisphere,  and  from 
south-east  to  north-west  in  the  southern.  The  regions  on  the  equator  being 
more  heated  than  the  surrounding  parts,  the  air  resting  on  them  is  rarefied, 
and  rising  flows  over  the  cooler  masses  towards  the  poles,  while  cold  air  from 
the  latter  rushes  in  below  to  supply  its  place.  Were  the  earth  stationary,  the 
trade  winds  would  be  due  north  on  one  side  of  the  equator,  and  due  south  on 
the  other.  The  earth's  diurnal  revolution,  however,  from  west  to  east,  mod- 
ifies these  directions  so  far  as  to  make  the  north  wind  north-east  and  the 
south  wind  south-east. 

The  trade  winds  are  of  great  service  to  mariners,  enabling  them  to  make 
certain  voyages  (for  instance,  from  the  Canaries  to  the  northern  coast  of 
South  America)  with  great  rapidity,  and  almost  without  touching  a  sail.  The 
zone  in  which  they  prevail  is  noted  for  its  transparent  atmosphere,  its  uni- 
formity of  temperature,  and  general  peaceful  aspect ;  whence  it  has  been 
called  by  the  Spaniards  "  the  sea  of  the  ladies  ". 

1020.  Periodical  Winds  are  such  as  blow  regularly  in 
the  same  direction  at  a  certain  season  of  the  year  or  hour 
of  the  day.    The  monsoon,  the  simoom,  and  the  land  and 
sea  breezes,  are  periodical  winds. 

The  monsoons  are  modifications  of  the  trade  winds,  which  sweep,  some- 
times with  great  violence,  over  the  Indian  Ocean  and  the  whole  of  Hindostan. 
For  six  months  they  blow  from  a  certain  quarter,  and  for  the  next  six  months 
from  the  opposite  one,  owing  to  the  change  in  the  sun's  position,  and  conse- 
quently in  the  heat  received  at  a  given  point. 

The  simoom,  originating  in  the  deserts  of  Asia  and  Africa,  is  distinguished 
by  its  scorching  heat  and  the  fine  sand  it  carries  with  it,  raised  from  the 
parched  surfaces  it  traverses.  The  simoom  from  the  Desert  of  Sahara, 
sweeping  over  the  intervening  regions,  finally  reaches  the  northern  shore  of 
the  Mediterranean,  and  is  there  called  the  Sirocco. — During  the  continuance 
of  this  hot  and  deadly  wind,  the  animal  and  the  vegetable  creation  droop 
with  excessive  exhaustion ;  travellers  on  the  desert  save  their  lives  only  by 
throwing  themselves  down  with  their  faces  in  the  sand. 

Land  and  sea  breezes  are  produced  by  the  unequal  heating  of  land  and 

kinds  of  -winds  are  there  ?  Name  them.  1019.  "What  are  Constant  Winds  ?  What 
are  the  most  noted  constant  winds?  Where  do  the  trade  winds  blow,  and  in  what 
direction  ?  Explain  the  origin  of  the  trade  winds.  How  do  they  benefit  mariners  ? 
What  do  the  Spaniards  call  the  region  in  which  th»y  prevail,  and  why  ?  1020.  What 
are  Periodical  Winds  ?  Mention  some.  Describe  the  monsoons.  Where  does  the 
Biniooin  originate,  and  by  what  is  it  distinguished  ?  What  is  the  Sirocco  ?  What  is 


WINDS.  403 

water.  During  the  day,  the  land  receives  more  heat  than  the  adjacent  ocean, 
the  rarefied  air  in  contact  with  it  rises,  and  a  gentle  breeze  sets  in  from  the 
sea  about  nine  in  the  morning,  which  gradually  increases  to  a  brisk  gale  in 
the  middle  of  the  day.  About  3  p.  M.  it  begins  to  subside,  *and  is  followed  in 
the  evening  by  a  land  breeze,  which  blows  freshly  through  the  night :  for 
after  sunset  the  land  rapidly  parts  with  its  heat  by  radiation,  and  the  air 
resting  on  it,  becoming  cooler  than  that  on  the  ocean,  rushes  to  supply  the 
place  of  the  latter  when  it  rises  in  consequence  of  being  rarefied. 

1021.  Variable  "Winds  are  those  which  are  irregular  as 
to  time,  direction,  and  force,  seldom  continuing  to  blow  for 
many  days  together.    They  prevail  chiefly  in  the  temperate 
and  frigid  zones,  the  winds  of  the  torrid  zone  being  for  the 
most  part  constant  or  periodical. 

1022.  Hurricanes. — Hurricanes  are  storms  that  revolve 
on  an  axis,  while  at  the  same  time  they  advance  over  the 
earth's  surface. 

Hurricanes  are  distinguish  eel  by  their  tremendous  velocity  and  great  ex- 
tent. They  are  often  500  miles  in  diameter,  and  sometimes  much  more.  In 
the  southern  hemisphere  they  always  revolve  in  the  same  direction  as  the 
hands  of  a  watch ;  in  the  northern  hemisphere,  in  the  opposite  direction. 
There  are  three  hurricane  regions ;  the  West  Indies,  the  Indian  Ocean,  and 
the  China  Sea.  In  the  last  they  are  called  Typhoons. 

1023.  Tornadoes. — Tornadoes,  or  Whirlwinds,  are  as 
violent  as  hurricanes,  but  more  limited  in  extent.     They 
are  rarely  more  than  a  few  hundred  yards  in  breadth  and 
twenty-five  miles  in  length.     Though  lasting  but  a  few  sec- 
onds in  a  given  place,  they  are  frequently  most  disastrous 
iil  their  effects,  prostrating  forests,  overturning  buildings, 
and  ravaging  the  whole  face  of  the  country. 

1024.  Water-spouts. — A  Water-spout  is  a  phenomenon 
frequently  observed  at  sea,  consisting  of  a  column  of  water 
raised  sometimes  to  the  height  of  a  mile  and  tapering  from 
each  end  towards  the  centre.     It  is  supposed  by  some  to 
be  produced  by  a  whirlwind  of  great  intensity ;  by  others 
it  is  attributed  to  electrical  influences. 

the  effect  of  the  simoom  on  the  animal  world  ?  When  do  land  and  sea  breezes  blow, 
and  how  are  they  produced  ?  1021.  What  are  Variable  Winds  ?  Where  do  they 
chiefly  prevail ?  1022.  What  are  Hurricanes?  By  what  are  they  distinguished?  In 
what  direction  do  they  revolve?  Name  the  three  hurricane  regions.  1023.  What 
are  Tornadoes?  Describe  their  effects.  1024.  What  is  a  Water-spout?  By  what  is  ' 
it  produced  ?  Give  an  account  of  the  way  in  which  it  ia  formed.  1025.  What  does 


404  METEOEOLOGT. 

Water-spouts  are  formed  as  follows  : — From  a  dark  cloud  a  conical  pillar 
is  seen  to  descend  with  its  point  downward.  As  it  approaches  the  water,  the 
latter  becomes  violently  agitated,  and  a  similar  column  rises  from  it,  point 
upward.  The  two  finally  unite,  forming  a  continuous  column  from  the  cloud 
to  the  water.  After  remaining  joined  for  a  time,  they  again  separate  into 
two  columns,  one  of  which  is  drawn  up  into  the  cloud,  while  the  other  pours 
down  in  the  form  of  heavy  rain.  Sometimes  the  two  columns  are  dispersed 
before  a  junction  is  effected. 

1025.  ATMOSPHERIC  MOISTURE.  —  The  atmosphere  al- 
ways contains  more  or  less  moisture,  derived  from  the 
earth's  surface,  particularly  those  portions  of  it  that  are 
covered  with  water,  by  the  process  of  evaporation.  When 
the  air  contains  as  much  moisture  as  it  is  capable  of  holding 
at  any  given  temperature,  it  is  said  to  be  saturated. 

The  higher  the  temperature  of  air,  the  more  moisture  it  is  capable  of  re- 
ceiving. At  32°  F.,  it  will  hold  only  »/160  of  its  own  weight  of  watery  vapor ; 
while  at  113°  it  will  receive  eight  times  as  much,  or  J/2o  of  its  own  weight. 

1026.  The  earth  gives  out  incredible  quantities  of  moisture  by  evaporation. 
Experiments  prove  that  an  acre  of  ground  apparently  parched  by  the  sun 
sends  forth  into  the  air  over  3,000  gallons  of  water  in  24  hours.  Of  course 
much  greater  quantities  are  evaporated  from  a  moist  soil  and  from  surfaces 
covered  with  water. 

1027.  The  Hygrometer. — The  amount  of  moisture  in  the 
atmosphere  is  ascertained  with  an  instrument  called  the 
Hygrometer.  Hygrometers  are  made  on  different  principles. 

In  some,  the  degree  of  humidity  is  indicated  by  the  elongation  of  a  hair, 
a  fibre  of  whalebone,  or  some  other  animal  substance  which  readily  absorbs 
moisture  and  is  increased  in  length  by  so  doing.  In  others,  it  is  shown  by 
the  increase  of  weight  in  some  substance  that  absorbs  moisture,  such  as 
sponge,  cotton,  or  potash.  In  the  more  delicate  instruments,  the  degree  of 
moisture  is  shown  by  the  greater  or  less  facility  with  which  it  is  condensed 
from  the  air  in  the  form  of  dew  on  a  cold  surface.  The  more  moisture  in  the 
air,  the  less  cold  will  be  required  to  condense  it  into  dew. 

1028.  Fog — Clouds. — When  the  air  is  cooler  than  the 
earth,  the  moisture  imparted  to  it  in  the  manner  just  de- 
scribed is  partially  condensed  and  thus  rendered  visible, 
forming  either  fog  or  clouds.   The  only  difference  between 
the  two  is  in  their  height.     When  the  condensation  takes 

the  air  always  contain?  When  is  it  said  to  be  saturated  f  On  what  does  the  amount 
of  moisture  that  air  can  receive  depend  ?  1026.  How  much  moisture  does  the  earth 
give  out  by  evaporation  ?  1027.  What  is  the  Hygrometer  ?  Mention  the  different 
principles  on  which  the  hygrometer  is  made.  1028.  When  are  Fog  and  Clouds 


CLOUDS.  405 

place  near  the  earth's  surface,  fog  is  the  result ;  when  in  the 
upper  regions  of  the  atmosphere,  clouds. 

1029.  Kinds  of  Clouds. — Clouds  are  divided  into  dif- 
ferent classes,  the  principal  of  which  are  the  Nimbus,  the 
Cumulus,  the  Stratus,  and  the  Cirrus. 

The  Nimbus,  or  rain-cloud,  is  a  dense  mass  of  vapor,  of  a  leaden  gray  or 
blackish  color,  with  a  lighter  tint  on  its  edges. — The  Cumulus  has  the  appear- 
ance of  many  dense  whitish  clouds  piled  up  one  on  another ;  or  of  a  vast  hem- 
isphere with  its  base  on  the  horizon,  and  peak  rising  above  peak,  looking 
like  huge  hills  of  snow  when  illumined  by  the  sun.  The  cumulus  may  be 
called  the  cloud  of  day,  and  is  an  indication  of  fair  weather. — The  Stratus 
consists  of  a  number  of  horizontal  layers  of  cloud,  not  very  far  removed  from 
the  earth's  surface.  Forming  at  sunset  and  disappearing  at  sunrise,  it  may 
be  called  the  cloud  of  night. — The  Cirrus  (called  caffs  tail  by  sailors)  is  a 
fleecy  cloud,  composed  of  thin  feathery  filaments  disposed  in  every  variety 
of  form.  The  cirrus  is  the  highest  of  all  clouds,  frequently  reaching  an  alti- 
tude of  from  three  to  five  miles.  It  is  no  doubt  often  composed  of  snow- 
flakes,  as  the  temperature  of  the  regions  in  which  it  floats  must  be  cold 
enough  to  freeze  the  watery  particles. 

1030.  DEW. — When  the  moisture  of  the   atmosphere 
comes  in  contact  with  an  object  colder  than  itself,  it  is  con- 
densed and  deposited  on  the  surface.     This  is  the  way  in 
which  Dew  is  formed. 

A  glass  of  ice-water  on  a  warm  day  is  almost  immediately  covered  with  a 
fine  dew.  So,  in  winter,  when  a  number  of  persons  are  in  a  warm  room,  the 
moisture  imparted  to  the  air  by  their  breath  is  condensed  on  the  window- 
panes  by  the  cold  air  without,  and  then  sometimes  frozen,  giving  them  a 
beautiful  frosted  appearance. — Just  so,  in  the  evening,  when  objects  on  the 
earth's  surface  are  cooled  down  by  radiation,  the  moisture  of  the  atmosphere 
is  deposited  on  them  in  the  form  of  dew. 

1031.  Dew  is  never  abundant  except  during  calm  serene  nights.  It  is 
generally  more  plentiful  in  spring  and  autumn  than  in  summer,  because  the 
difference  between  the  temperature  of  day  and  night  is  greater  in  those  sea- 
sons. The  quantity  of  dew  precipitated  on  different  bodies  depends  much 
upon  their  nature.  Thus  grass  and  leaves  will  frequently  be  found  glistening 
with  crystal  drops  at  sunrise,  when  gravelled  walks,  stones,  wood-work,  and 
metallic  surfaces,  are  comparatively  dry — another  striking  proof  of  the  wis- 
dom with  which  Providence  orders  the  economy  of  nature. 

1032.  Frost  is  nothing  more  than  frozen  dew. 

formed  ?  "What  is  the  difference  between  them  ?  1029.  Name  the  different  kinds  of 
clouds.  Describe  the  Nimbus.  The  Cumulus.  The  Stratus.  The  Cirrus.  1030.  Un- 
der what  circumstances  is  Dew  formed  ?  What  familiar  instances  of  the  formation 
of  dew  are  mentioned?  1031.  When  is  dew  most  abundant?  How  does  its  orecipi- 


406  METEOROLOGY. 

1033.  RAIN. — Rain  is  water  taken  up  by  the  air  in  the 
form  of  vapor  and  returned  to  the  earth  in  drops. 

When  two  masses  of  damp  air  differing  considerably  in  temperature  are 
mingled,  they  become  incapable  of  retaining  the  same  amount  of  moisture 
which  they  held  while  they  remained  apart.  The  excess  is  precipitated  in 
the  form  of  rain,  the  vesicles  of  vapor  under  the  influence  of  mutual  attrac- 
tion blending  together  and  forming  drops. 

Some  parts  of  the  earth  never  have  any  rain,  vegetation,  when  it  exists 
at  all,  being  supported  entirely  by  dew.  This  is  the  case  with  Peru,  the 
Desert  of  Sahara,  portions  of  Arabia  and  Egypt,  and  extensive  districts  in 
Central  Asia.  In  other  parts,  for  example  Guiana,  it  rains  almost  con- 
stantly. The  Island  of  Chiloe  has  a  rather  moist  climate ;  the  people  there 
have  a  current  saying,  that  it  rains  six  days  in  the  week  and  is  cloudy  the 
seventh.  '•*  '£ 

1034.  SNOW. — Snow  consists  of  the  watery  particles  of 
the  atmosphere  frozen  for  the  most  part  in  a  crystalline  form. 

Viewed  through  a  microscope,  snow-flakes  exhibit  forms  of  great  beauty 
and  endless  variety.  Between  six  and  seven  hundred  different  forms  have  been 
distinguished,  many  of  them  belonging  to  the  six-sided  system  df  crystals. 

Snow  of  a  beautiful  crimson  color  and  a  delicate  green  has  been  found  in 
different  parts  of  the  world.  These  tints  are  due  to  minute  plants  or  animal- 
cules in  different  stages  of  development. 

1035.  HAIL. — Hail  consists  of  globules  of  ice  formed  in 
the  atmosphere  by  the  congelation  of  its  moisture  and  pre- 
cipitated to  the  earth. 

Hail  is  produced  by  an  intense  degree  of  cold  in  the  atmosphere,  and  is 
generally  accompanied  with  electrical  phenomena.  It  is  rare  at  the  level  of 
the  sea  within  the  tropics,  and  in  high  latitudes  is  totally  unknown,  being 
most  abundant  in  temperate  climates.  Hail-storms  seldom  continue  a  quarter 
of  an  hour,  but  while  they  last  large  quantities  of  ice  falL  The  stones  are 
generally  pear-shaped,  and  frequently  weigh  ten  or  twelve  ounces.  Masses 
weighing  6,  8,  and  even  14  pounds,  have  been  known  to  fall. 

tation  show  the  goodness  of  Providence  ?  1032.  What  is  Frost  ?  1033.  What  is  Kain  ? 
How  is  rain  formed  ?  What  parts  of  the  earth  never  have  any  rain  ?  Where  doea 
It  rain  almost  constantly  ?  1034.  Of  what  does  Snow  consist  ?  What  is  the  form  of 
snow-flakes  ?  Of  what  color  has  snow  sometimes  been  found  ?  How  is  this  account- 
ed for  ?  1035.  Of  what  does  Hail  consist  ?  How  is  it  produced  ?  Where  is  it  most 
frequent  ?  What  is  the  shape  of  hail -stones  ?  How  large  have  they  been  known 
to  Ml  ? 


FIGURES 


FOR  the  convenience  of  the  pupil  during  recitation,  the  Figures 
to  which  reference  is  made  by  letters  are  here  reproduced.  The 
numbers  correspond  with  those  of  the  text. 


Fig.  1. 


Fig.  8. 


Fig.  13. 


Fig.  16. 


Fig.  14. 


Fig.  15. 


Fig.  18. 
o 


Fig.  20. 
o 


Fig.  21. 


Fig.  22. 


408 


NATURAL  PHILOSOPHY. 


Fig.  24 


Fig.  32. 


Fig.  2& 
E  B 


Fig.  81. 


Fig.  83. 
B  C 


Fig.  84. 


Fig.  36.  Fig.  87. 


Fig.  88. 


410 


NATURAL  PHILOSOPHY. 


Fig.  53. 


Fig.  54. 
A 


FIGURES. 


Fig.  58. 


Fig.  75. 


Fig.  60. 
G 


Fig, 

^ 

P 


Fig.  74. 


v\    r>       -f 

^^/Httk^       / 

—7  X^l-7      /* 


'G  :X*x          O 
^>          V*> 


^ 


Fig.  T8. 


Fig.  85. 


412  NATURAL  PHILOSOPHY. 

Fig.  86.  Fig.  87. 


Fig.  91. 


©  0 


Fig.  95. 


Fig.  96. 


Fig.  98. 


Fig.  99. 
A . 3 


FIGURES. 


413 


Fig.  100. 


Fig.  103. 


Fig.  102. 


Fig.  104. 


Figf.  105. 


Fig.  106. 


Fig.  108. 


414 


NATURAL   PHILOSOPHY. 


Fig.  109. 


Fig.  111. 


Fipr.  113. 


Fig.  115. 


Fig.  112. 


Fig.  114. 


Fig.  116. 


Fig.  117. 


FIGURES.  415 

Fig.  118.  Fig.  119. 


Fig.  128. 


Fig.  122. 


Fig.  124. 


Fig.  126. 


416  NATUKAL  PHILOSOPHY. 

Fig.  12T.  Fig.  129. 


Fig.  131. 


Fig.  134 


Fig.  136. 


Fig.  137. 

D 


FIGURES. 


417 


Fig.  139. 


Fie.  142. 


Fig.  143. 
E 


IS* 


418  NATURAL   PHILOSOPHY. 

Fig.  145. 

Fig.  146.  Fig.  149. 


Fia  151. 


Fig.  152. 


FIGURES. 


419 


Fig.  153. 


Fig.  160. 


Fig  163. 


Fig.  164. 


-is 

Fig.  165. 


Fig.  166. 


Fig.  167. 


420 


NATURAL   PHILOSOPHY. 

Fig.  168. 


Fig.  170. 


Fig.  171. 


FIGURES. 


421 


Fig.  1T& 


Fig.  174. 


Fig.  1T7. 


Fig.  179. 


422 


NATUEAL   PHILOSOPHY. 

Fig.  1ST. 


Fig.  182. 


Fig.  190. 


Fig.  193. 


Fisr.  192. 


423 


Fig.  198. 


Fig.  196. 


Fig.  2CO. 


424  NATURAL   PHILOSOPHY. 

Fig.  201.  Fig.  203.  Fig.  204. 


FIGUEES. 


425 


Fig.  213. 


426 


NATURAL  PHILOSOPHY. 

Fig.  215. 


FIGUKES. 


427 


Pig.  222. 


428 


NATURAL  PHILOSOPHY. 


Fig.  224. 


Fig.  228. 


Fig.  231. 
A 


FIGURES. 


429 


Fig.  232. 


Fig.  288. 


Fig.  236. 


Fig.  237. 


..- 


Fig.  235. 


Fig.  238 


Fig.  239. 


430 


NATURAL  PHILOSOPHY. 


Fig.  241. 


C     B 


Fig.  242. 


Fig.  244. 


Fig.  248. 
A 


Fig.  250. 


Fig.  25a 


FIGURES. 


431 


Fig.  256. 


Fig.  25& 


C         1) 


432 


NATURAL  PHILOSOPHY. 


Fig.  259. 


FV" 


Fig.  260. 


Fig.  26L 


Fig.  264. 


Fig.  265. 


FJg.  2«6. 


434 


NATURAL  PHILOSOPHY. 


Fig.  274. 


* — --ID 


Fig.  285. 


-«*. 

k 


Fig.  286. 


Fig.  287. 


435 


Fig.  299. 


Tig.  302. 


Fig.  SOO. 


Fig.  297. 


Fig.  801. 

6    b    b    5 


vww 

a    a    a    a   & 

unAfu 

a       a       a       a 


Fig.  303. 

Is 


J C 


436 


NATURAL   PHILOSOPHY. 


Fig  316. 


FIGURES. 


437 


Fig.  323. 


Fig.  328. 


.  o^ 


NATURAL  PHILOSOPHY. 

Fig.  328. 


439 


Fig.  835. 


410 


INDEX. 


[THE  FIGITEEB  BEFEB  TO  PAGES,  NOT  TO  SECTIONS.] 


A. 

Aberration,  chromatic,  259. 

Acoustics,  defined,  274. 

Actinism,  257. 

Action,  defined,  42.  Equal  to  reaction,  43. 

Adhesion,  defined,  21.  Experiments  il- 
lustrative of,  21. 

Adjutage,  different  forms  of,  155. 

Aeriform  bodies,  defined,  8. 

Affinity,  chemical,  9. 

A  gents,  defined,  7. 

Air,  composition  of,  9.  Tends  to  stop 
motion,  36.  Resistance  of  the,  54;  ef- 
fect of,  59,  62.  Exists  in  every  sub- 
stance, 166.  Is  impenetrable,  166.  Is 
compressible,  167.  Is  elastic,  167.  Has 
weight,  168.  Density  of,  at  different 
levels,  174.  Effects  of  its  rarity,  175. 
Earefied  by  heat,  175.  Navigation  of 
the,  177.  Essential  to  life  and  combus- 
tion, 184  Supports  a  column  of  water 
from  32  to  34  feet  high,  187.  A  non- 
Conductor  of  heat,  201. 

Air-gun,  the,  168. 

Air-pump,  the,  177.  Single-barrelled, 
178.  Double-barrelled,  179.  Experi- 
ments with,  180. 

Alfred  the  Great,  his  mode  of  measuring 
time,  126. 

Amazon,  the,  its  fall,  156.  Its  discharge, 
156. 

Anemometer,  the,  401. 

Angle,  defined,  35.  Vertex  of,  35.  Eight, 
85.  Obtuse,  35.  Acute,  35.  Visual,  264. 


Anode,  defined,  324. 

Aphelion,  of  a  planet,  374. 

Apogee,  889. 

Apple-cutter,  the,  ISO. 

Aqueducts,  of  the  ancient  Eomans,  13a 

Arc,  defined,  34. 

Arch,  the  voltaic,  32a    Celestial,  384. 

Archimedes,  reasoned  by  induction,  10. 
Explained  the  properties  of  the  lever, 
94.  Discovered  the  leading  principles 
of  specific  gravity,  146.  His  screw,  162. 
Fired  the  Eoinan  fleet  with  concave 
mirrors,  194. 

Aristotle,  his  doctrine  respecting  falling 
bodies,  54. 

Armature,  of  magnets,  335. 

Ascending  Bodies,  59.  Height  reached 
by,  60. 

Asteroids,  the,  392. 

Astronomy,  defined,  368.  Fundamental 
facts  of,  369. 

Athermanous  substances,  defined,  206. 

Atmosphere,  the,  166.  Pressure  of,  169, 
174 ;  proposed  mode  of  transmitting 
mails  by  the,  182.  How  heated  by  the 
sun,  204.  Moisture  of,  404 

Atomic  Theory,  17. 

Atoms,  what  they  are,  17. 

Attraction,  of  gravitation,  20.  Molecu- 
lar, 21.  Capillary,  146.  Between  float- 
ing bodies,  149.  Electrical,  289,  290. 
Magnetic,  337. 

Atwood's  Machine,  56,  57. 

Aurora  borealis,  303. 

Awis,  of  a  sphere,  71. 


INDEX. 


441 


B. 

Bacon,  Roger,  invented  spectacles,  263. 

Balance,  the,  96.    Of  a  watch,  128. 

Ballistic  Pendulum,  the,  64. 

Balloons,  why  they  rise,  53.  Rose,  why 
they  lose  their  buoyancy,  150.  Inven- 
tion of,  176. 

Barometer,  the,  171.  The  wheel,  172. 
Its  use  as  a  weather  guide,  172. 

Barometer  gauge,  the,  180. 

Base,  of  a  body,  72. 

Battery,  the  electrical,  300.  The  galvan- 
ic, 319;  the  trough,  320;  Smee's,  320; 
Daniell's,  321 ;  Grove's,  322 ;  Bunsen's, 
822 ;  theory  of,  323.  The  thermo-elec- 
tric, 332.  The  relay,  861. 

Bay  ofFundy,  tides  of  the,  157. 

.Bea™,  a,  of  light,  230. 

Bell,  vacuum,  183.    Electrical,  301. 

Bellows,  hydrostatic,  137.  Principle  of 
the  common,  170. 

Bladder-glass,  the,  181. 

Blindness,  cause  of,  263. 

Boats,  propulsion  of,  ICO.    Shape  of,  161. 

Bode,  his  law,  376, 

Body,  a,  what  it  is,  7.  A  simple,  8.  A 
compound,  8.  The  simple  bodies,  8. 
When  it  stands  and  falls,  73. 

Boilers,  how  made,  225. 

Boiling,  process  of,  explained,  203. 

Bottle,  thousand  grain,  141. 

Bottle  Imps,  167, 182. 

Breadth,  defined,  12. 

Breathing  process  of,  explained,  170. 

Breezes,  land  and  sea,  402. 

Brewster,  Sir  David,  invented  the  kalei- 
doscope, 241. 

British  Channel,  tides  of  the,  157. 

Brittleness,  defined,  23. 

Buckets,  of  a  wheel,  158. 

Bu/on,  his  experiment  wit-h  concave 
mirrors,  194. 

Burning,  process  of,  195. 

Burning  glasses,  243,  251. 


C. 

Calms,  region  of,  402. 
Caloric,  192. 

Camera  obscura,  266.    Draughtsman's, 
267.    Daguerreotypist's,  267. 
19" 


|  Canals,  principle  of  their  construction, 
134. 

Cancer,  tropic  of,  388. 

Capillary  Attraction,  what  it  is,  146. 
Cause  of,  146.  Familiar  examples  of, 
147.  Laws  of,  148,  Interesting  facts 
connected  with,  149. 

Capricorn,  tropic  of,  388. 

Capstan,  the,  described,  105. 

Carbon,  combines  with  oxygen  to  pro- 
duce animal  heat,  196. 

Carbonic  acid,  found  at  the  bottom  of 
wells,  140. 

Cathode,  defined,  324. 

Catoptrics,  236. 

Centre,  of  gravity,  70.  Of  magnitude,  7t 
Of  motion,  71. 

Centre  of  gravity,  what  it  is,  70.  How 
to  find  it,  71.  In  man,  77.  Tends  to 
get  as  low  as  possible,  78. 

Centrifugal  force,  defined,  37.  Exam- 
ples of,  38,  39.  Law  of  the,  39.  Its  ef- 
fect on  revolving  bodies,  89.  Apparatus 
to  illustrate  the,  40.  Gave  its  form  to 
the  earth,  41. 

Centripetal  Force,  defined,  3,7. 

Ceres,  when  discovered,  375. 

Chemical  action,  a  source  of  heat,  195. 

Chemical  affinity,  9. 

Chemistry,  defined,  9. 

Chinese,  the,  early  acquainted  with  gun- 
powder, 63.  First  used  the  magnet  tn 
navigation,  842. 

Chord,  what  it  is,  285. 

Chords,  vocal,  286. 

Chromatics,  254. 

Chronometers,  how  perfect,  127. 

Circle,  defined,  34.  Simple  galvanic,  319. 
The  arctic,  389.  The  antarctic,  3S9. 

Circumference,  defined,  34.  How  divid- 
ed, 35. 

Cirrus,  the,  defined,  405. 

Clepsydra,  the,  126.  Described,  153.  In- 
vented  by  Ctesibius,  187. 

Clocks,  how  regulated,  68.  History  of, 
126.  Pendulum  applied  to,  126.  Work* 
of,  127.  Electro-magnetic,  364. 

Clouds,  how  formed,  404.     Kinds  of,  405. 

Coat,  choroid,  262.    Sclerotic,  262. 

Cogs,  what  they  are,  123. 

Cohesion.,  defined,  21. 

Cold,  what  it  is,  192. 


442 


INDEX. 


Colors,  the  primary,  255.  Difference  of, 
explained,  256.  Complementary,  257. 

Columbus,  his  discovery  respecting  the 
variation  of  the  compass,  343.  Saved 
himself  and  his  men  by  predicting  an 
eclipse,  396. 

Combustion,  what  it  is,  195.  Produces 
most  of  our  artificial  light,  231. 

Comets,  what  they  consist  of,  397.  Their 
orbits,  397.  Their  velocity,  397.  Their 
number,  39S. 

Compass,  land  or  surveyor's,  341.  Mari- 
ner's, 842.  Boxing  the,  342. 

Compressibility,  19,    Of  air,  20. 

Concord,  285. 

Condensation,  212.    Of  steam,  218. 

Condenser,  the,  1S4.  Of  the  steam-en- 
gine, 222. 

Conductometer,  the,  199. 

Conductors,  of  heat,  199.  Of  electricity, 
293. 

Conjunction,  879. 

Constellations,  386,  899. 

Convection,  of  heat,  202. 

Copernicus,  revived  the  true  theory  of 
the  universe,  370. 

Cornea,  the,  261.    Use  of,  262. 

Couronne  des  tosses,  the,  320. 

Crank,  the,  124. 

Crown-wheel  and  pinion,  123. 

Crutch,  the,  of  an  escapement,  127. 

Ctesibius,  invented  the  lifting-pump,  186. 
Invented  the  clepsydra,  187.  Supposed 
to  have  invented  the  water-organ,  284. 

Cumulus,  the,  described,  405. 

£up,  Tantalus's,  186.  The  phosphorus,  305. 

Cupping-glasses,  principle  of,  175. 

Curb,  of  a  watch,  129. 

Curves,  magnetic,  338. 

Cylinder,  defined,  36. 

D. 

Daguerreotype  process,  the,  268. 

Dams,  should  increase  in  strength  at  the 
base,  136. 

Dead-point,  the,  of  a  crank,  125. 

Density,  19.    In  optics,  246. 

Descartes,  advanced  the  undulatory  the- 
ory of  light,  229. 

Dew,  how  formed,  405. 

Diagonal,  dcflnod,  85. 

Diamaynetism,  867. 


I  Diameter,  defined,  84 

Diathermanous  substances,  defined,  20fl, 

Dionysius,  ear  of,  281. 

Dioptrics,  246. 

Dip,  magnetic,  340. 

Direction,  line  of,  71. 

Discharger,  the  jointed,  298.  The  uni- 
versal, 298. 

Discord,  285. 

Dispersion,  01  light,  259. 

Distillation,  process  of,  described,  212. 

Diving-bell,  the,  166. 

Divisibility,  defined,  17.  Instances  of,  18. 

Double  cone,  may  be  made  to  roll  up  an 
inclined  plane,  80. 

Draft,  how  produced  in  a  chimney,  176. 

Driver,  the,  120. 

Drum,  the,  2S3. 

Ductility,  defined,  26.  Of  platinum,  26. 
Of  gold,  26.  Of  glass,  26. 

Du  Fay,  his  theory  respecting  electrici- 
ty, 291. 

E.        V. 

Ear,  the  human,  287. 

Earth,  the,  owes  its  form  to  the  centrifu- 
gal force,  41.  Magnetic  poles  of,  344. 
Form  of,  382.  Motions  of,  3S3.  Orbit 
of,  384.  Phases  of,  to  the  moon,  390. 
How  it  would  look  from  the  fixed  stars, 
895. 

Ear-trumpet,  the,  280. 

Echoes,  279. 

Eclipse,  of  the  sun,  how  produced,  395. 
Annular,  396.  Of  the  moon,  396. 

Ecliptic,  the,  385.    Obliquity  of,  355. 

Eel,  the  Surinam,  316. 

Elasticity,  defined,  24.  Perfect,  24.  Be- 
longs to  hard  solids,  24.  Of  steel,  24 
A  limit  to,  25. 

Electricity,  a  source  of  light,  232.  What 
it  is,  2S9.  Sources  of,  290.  Developed 
by  friction,  290.  Vitreous,  or  positive, 
291.  Resinous,  or  negative,  291.  Na- 
ture of,  291.  Conduction  of,  293.  Path 
of,  294.  Velocity  of,  294.  Machines  for 
developing,  294;  experiments  with,  301. 
Mechanical  effects  of  the  passage  ol,  304. 
From  steam,  310.  Atmospheric,  310. 
Voltaic,  316.  Difference  between  fric- 
tional  and  voltaic,  324  Developed  by 
heat,  832.  Connection  between  mag- 


INDEX. 


443 


netism  and,  852.  Developed  by  mag- 
netism, 366. 

Electrics,  293. 

Electrodes,  what  they  are,  323. 

Electro-magnetism,  defined,  349.  As  a 
motive  power,  357. 

Electro-magnets,  356, 35T. 

Electro-metallurgy,  326. 

Electrometer,  the,  309.  The  quadrant, 
309. 

Electrophorus,  the,  308. 

Electroscope,  the,  308. 

Electrotyping,  process  of,  described,  827. 

Elements,  sixty-two  in  number,  8.  Di- 
vided into  -metals  and  non-metallic,  8. 
The  non-metallic  enumerated,  9. 

Endless  Band,  121. 

Endosmose,  150. 

Engine,  denned,  83.  Atmospheric,  219. 
Steam,  219.  Hero's,  219.  Marquis  of 
Worcester's,  220.  Savery's,  221.  New- 
comen's,  222.  Watts',  222.  The  low 
pressure,  226.  The  high  pressure,  226. 
The  locomotive,  226. 

Equator,  the  magnetic,  341.  The  celes- 
tial, 385. 

Equilibrium,  stable  and  unstable,  79. 

Equinoctial,  the,  885. 

Equinoxes,  385.    Precession  of  the,  386. 

Escapement,  of  clocks,  127.  Of  watches, 
128. 

Esquimaux,  why  they  thrive  on  fat,  196. 

Evaporation,  211. 

Exosmose,  150. 

Expansibility,  19.    Of  air,  20. 

Expansion,  207. 

Experiment,  what  it  consists  in,  10. 

Extension,  defined,  12. 

Eye,  the,  260.  Parts  of,  261.  Adaptation 
of,  265. 

F. 

Fahrenheit,  his  thermometrical  scale,  214. 

Falling  bodies,  velocity  of,  54.  Law  of, 
56.  Eules  relating  to,  58. 

Faraday,  his  theory  respecting  electric- 
ity, 292. 

Fata  morgana,  248. 

Figure,  defined,  12. 

Fire,  St  Elmo's,  311. 

Fire-alarm,  electro-magnetic,  865. 

Fire-balls,  812. 


Fire-engine,  principle  of  the,  183 

Fire-escape,  the,  107. 

Fire-house,  the  electrical,  307. 

Fish,  how  they  rise  and  sink  in  water, 
145.  Electrical,  316. 

Flame,  how  produced,  195. 

Float-boards,  158. 

Fluids,  embrace  liquids  and  aeriform  bod- 
ies, 8.  Difference  between  them  and 
solids,  8.  Non-elastic,  25.  Elastic,  25. 
Division  of  elastic,  165. 

Flute,  the,  principle  of,  283. 

Flyer,  the  electrical,  305. 

Flywheel,  the,  125. 

Focus,  the  principal,  242.  The  -v  irtual,  244. 

Fog,  how  formed,  404. 

Follower,  the,  120. 

Force,  defined,  26.  Striking,  31.  Cen- 
trifugal, 37.  Centripetal,  37. 

Forge-hammer,  the,  124. 

Fountains,  how  high  they  rise,  132.  Vac- 
uum, 181. 

Franklin,  his  theory  "respecting  electric- 
ity, 292.  Proved  lightning  to  be  pro- 
duced by  an  electric  discharge,  312. 
Invented  the  lightning-rod,  315. 

Friction,  what  it  is,  37,  85.  How  it  op- 
poses motion,  85.  Kinds  of,  85.  Slid- 
ing, converted  into  rolling,  86.  Laws 
of,  86,87.  Modes  of  lessening,  87.  Uses 
of,  88.  Of  one  wheel  on  another,  120. 
Of  water  against  the  sides  of  pipes,  155. 
Of  a  stream  against  its  banks,  156.  En- 
ables the  wind  to  produce  waves,  156. 
A  source  of  heat,  197.  A  source  of  elec- 
tricity, 290. 

Frost,  what  it  is,  406. 

Fulcrum,  what  it  is,  94 

Furnace,  of  a  steam-engine,  226. 

Fusee,  of  a  watch,  128. 

G. 

Galaxy,  the,  400. 

Galileo,  his  doctrine  respecting  falling 
bodies,  54.  Invented  the  pendulum,  67. 
First  made  a  practical  use  of  the  tele- 
scope, 272.  Established  the  truth  of  the 
Copernican  system,  371. 

Galle,  Dr.,  discovered  Neptnne,  394. 

Galleries,  whispering,  281. 

GuMtinl,  discovered  roltdfc  electricity^ 
817.  His  experiment,  817. 


444 


INDEX. 


Galvanism,  816.  (For  particulars,  see 
Voltaic  electricity.) 

Galvanometer,  the,  351.  "With  astatic 
needle,  352. 

Gamut,  the,  284. 

Gases,  what  they  are,  165.  Specific  grav- 
ity of,  how  found,  143.  Exhibit  endos- 
mose  and  exosmose,  150.  Conducting 
power  of,  201.  Expansion  of,  210. 

Gearing,  what  it  is,  121. 

Gioia,Flavio,  improved  the  compass,  342. 

Glottis,  the,  286. 

Governor,  the,  225. 

Gravitation,  defined,  20.  Circumstances 
attending  its  discovery,  47.  Facts  es- 
tablished respecting  it,  47.  Direction 
of,  48.  Laws  of,  49. 

Gravity,  terrestrial,  46.  Laws  for  the 
force  of,  49.  Sometimes  causes  bodies 
to  rise,  53.  Centre  of,  70.  Used  as  a 
motive  power,  81.  Specific,  139.  Ta- 
bles of  specific,  144. 

Guericke,  invented  the  air-pump,  177. 
His  famous  experiment,  178.  First 
contrived  an  electrical  machine,  295. 

Gunnery,  63. 

Gunpowder,  principle  on  which  it  acts, 
63.  Invention  of,  63.  Apparatus  for 
firing,  with  electricity,  808. 

Gutta  perchat  used  for  endless  bands,  121. 

H. 

Ilail,  its  disastrous  effects,  59.  How 
formed,  406. 

Hair-spring,  the,  of  a  watch,  128. 

Haloes,  what  they  are,  260. 

Hand-glass,  the,  180. 

Hardness,  defined,  22.  "Wanting  in  flu- 
ids, 22.  Of  various  solids  compared,  22. 

Harmony,  what  it  is,  2S5. 

Harp,  ^Eolian,  283. 

Heat,  what  it  is,  192.    Free,  or  sensible, 

192.  Latent,  192.    Theories  respecting, 
198.    Has  no  weight,  193.    Sources  of, 

193.  The  sun's,  194;  how  it  may  be 
increased,  194;   how  far  it  penetrates 
into  the  earth,  194.    Below  the  earth's 
surface,  194.      Produced  by  chemical 
action,  195.  Animal,  or  vital,  196.    Pro- 
duced by  mechanical  action,  196.  From 
friction,  197.     From  percussion,  197. 
Produced  by  electricity,  198.  Diffusion 


of,  198;  by  conduction,  199;  by  con- 
vection, 202 ;  by  radiation,  203.  Eadi- 
ant,  204;  law  of,  204;  reflection  of,  205; 
absorption  of,  206 ;  transmission  of,  206. 
Effects  of,  207.  Instruments  for  meas- 
uring, 213.  Specific,  216. 

Helix,  the,  355.  Magnetizing  power  of, 
855.  Itself  a  magnet,  365. 

Hemispheres,  the  Magdeburg,  178. 

Hero,  his  steam-engine,  219. 

Herschel,  his  telescope,  273.  Discovered 
Uranus,  375. 

Hiero,  golden  crown  of,  145. 

Hooke,  Dr.,  added  the  hair-spring  to  tho 
balance,  127. 

Horizon,  the  sensible,  384.  The  rational, 
884.  Poles  of  the,  885. 

Horse,  the,  strength  of,  S2. 

Horse-poicer,  defined.  84 

Humor,  aqueous,  261.    Yitreous,  261. 

Hurricanes,  403. 

Uuygens,  applied  the  pendulum  to  clock- 
work, 67.  Unfolded  the  undulatory 
theory  of  light,  229. 

Hydraulics,  defined,  152. 

Hydraulicon,  the,  284. 

Hydrogen,  the  lightest  substance  known, 
144.  Used  for  inflating  balloons,  176. 
Produces  musical  sounds,  284. 

Hydrometer,  the,  142. 

Hydrostatics,  defined,  180.  Law  of,  131. 
Hydrostatic  paradox,  137.  Itydrostatio 
bellows,  137.  Hydrostatic  press,  138. 

Hygrometer,  the,  404. 

I. 

Ice,  process  of  its  formation,  210. 
Iceland  spar,  exhibits  double  refraction, 

252. 

Image,  an,  what  it  is,  239. 
Impenetrability,  defined,  13.    Of  air,  13. 

Instances  of,  13. 
Incandescence,  213. 
Incidence,  angle  of,  46.    Equal  to  angle 

of  reflection,  46. 
Inclined  Plane,  the,  110.    Law  of,  110. 

Practical  applications  of,  111.    Law  of 

bodies  rolling  down,  111. 
Indestructibility,  defined,  13.    Instances 

of,  13.    Anecdote  illustrative  of,  18. 
InducUon,  electrical,  309.   Magnetic,  346. 


INDEX. 


445 


Inertia,  defined,  15.  Examples  of,  15. 
Experiments  illustrative  of,  15, 16.  Pro- 
portioned to  a  body's  weight,  17. 

Instruments,  optical,  266.  Stringed,  2S2. 
Wind,  283. 

Insulators,  294. 

Ifidium,  one  of  the  hardest  metals,  22. 
The  heaviest  known  substance,  144. 

Iris,  the,  261.    Use  of,  262. 

Iron,  great  tenacity  of,  23. 

J. 

Jar,  the  Leyden,  299. 

Juno,  when  discovered,  3T5. 

Jupiter,  velocity  of  light  ascertained  from 
the  eclipses  of  one  of  its  moons,  234.  Its 
seasons,  389.  Details  of  the  planet,  392. 

K. 

Kaleidoscope,  the,  241. 
Kepler,  his  laws,  377. 

I*. 

Lamp,  principle  on  which  it  burns,  147. 

Landes,  shepherds  of,  78. 

Lantern,  a  species  of  wheel,  123.  Origin 
of  the,  126.  The  magic,  271.  Phantas- 
magoria, 271. 

Larynx,  the,  286. 

Lava,  discharge  of,  accounted  for,  195. 

Leaves,  what  they  are,  122. 

Length,  defined,  12. 

Lens,  the  crystalline,  261. 

Lenses,  what  they  are,  246.  Classes  of, 
249.  Befraction  by  convex,  250.  Re- 
fraction by  concave,  251.  Achromatic, 
259. 

Le  Sage,  first  attempted  to  transmit  mes- 
sages by  electricity,  263. 

Level,  the  spirit,  134.    The  water,  185. 

Lever,  what  it  is,  94  Of  the  first  kind, 
94,  95.  Practical  applications  of  the, 
98, 100, 102.  The  bent,  99.  The  com- 
pound, 99.  Of  the  second  kind,  99.  Of 
the  third  kind,  101.  Perpetual,  104. 
Often  combined  with  the  screw,  115. 

Le  Verrier,  his  prediction  verified  by  the 
discovery  of  Neptune,  394. 

Life-boats,  principle  of,  144. 

Life-preservers,  principle  of,  144. 

Light,  what  it  is,  229.  Corpuscular  the- 
ory of,  229.  Undulatory  theory  of,  229. 


I  Sources  of,  231.  Of  the  sun,  232.  Of  the 
stars,  232.  Propagation  of,  232.  Veloc- 
ity of,  233.  Intensity  of,  at  different 
distances,  234.  Reflection  of,  236.  Re- 
fraction of,  245.  Laws  of  refracted,  246. 
Polarization  of,  253.  Dispersion  of,  259. 
The  electric,  328.  The  zodiacal,  373. 

Lightning,  312.    Effects  of,  313. 

Lightning  rod,  the,  813. 

Lights,  northern,  803. 

Line,  a  right,  defined,  34.  Parallel  linei 
defined,  34.  A  curve,  34.  The  neutral, 
of  a  magnet,  334. 

Liquefaction,  210. 

Liquids,  defined,  8.  How  they  differ  from 
solids,  130.  Have  little  cohesion,  181. 
Compressibility  of,  131.  Not  devoid  of 
elasticity,  131.  Pressure  of,  135.  Rule 
for  finding  their  pressure  on  the  bottom 
of  a  vessel,  137.  Specific  gravity  of,  141. 
Exhibit  endosmose  and  exosmose,  150. 
Flow  of,  through  orifices,  152.  Flow 
of,  in  pipes,  155.  Conducting  power  of, 
201.  Expansion  of,  209.  Converted 
into  vapor  by  heat,  211.  Good  con- 
ductors of  sound,  276. 

Living  Force,  81. 

Loadstone,  described,  334. 

Lock,  on  a  canal,  134. 

Locomotive,  the,  226. 

Lubricants,  87. 

Litngs-glass,  the,  181. 

Itt. 

Machines,  what  they  are,  83.  Can  not 
create  power,  88.  Law  of,  89.  Advan- 
tages of  using,  90.  All,  combinations 
of  the  six  mechanical  powers,  120.  Must 
be  regular  in  their  motion,  125.  For 
raising  water,  161.  Electrical,  294.  Cyl- 
inder, 295,  296.  Plate,  297.  Hydro- 
electric, 310.  Magneto-electric,  367. 

Magic  lantern,  the,  271. 

Magnetism,  defined,  333.  Theory  of,  343. 
Terrestrial,  344;  intensity  of,  345.  By 
induction,  346.  By  the  sun's  rays,  347. 
By  contact  with  a  magnet,  347.  De- 
veloped by  electricity,  349.  Connection 
between  electricity  and,  352. 

Magneto-electricity,  366.  Medical  use  of, 
367. 

Magnets,  what  they  are,  333.    Natural, 


146 


INDEX. 


834  Poles  of,  834,  837.  Power  of  nat- 
ural, 835.  Armed,  335.  Artificial,  335. 
Bar,  336.  Horse-slioe,  336.  Compound, 
836.  Power  of,  how  increased  and  di- 
minished, 337.  Attraction  of,  837 ;  law 
of,  338.  Polarity  of,  338.  Production 
of  artificial,  345. 

Magnifying  glasses,  251. 

Main-spring,  the,  of  a  watch,  128. 

Malleability,  defined,  25.  Of  the  metals, 
25,  26. 

Mariotte's  Law,  168. 

Mars,  details  of  the  planet,  891. 

Matter,  defined,  7.  Ponderable,  7.  Im- 
ponderable, 7.  Forms  of  ponderable,  7. 
Properties  of,  12. 

Mechanical  Powers,  the,  94. 

Mechanics,  defined,  26. 

Medium,  a,  what  it  is,  231.  A  uniform, 
231.  A  dense,  246.  A  rare,  246. 

Melody,  what  it  is,  285. 

Meniscus,  what  it  is,  249. 

Mercury,  details  of  the  planet,  381. 

Meridian,  the  magnetic,  340. 

Metals,  the  principal,  9.  Specific  gravity  of 
various,  144.  Precious,  how  tested,  145. 
Protection  of,  by  voltaic  electricity,  328. 

Meteorology,  defined,  401. 

Melius,  supposed  to  have  invented  the 
telescope,  272. 

Microscope,  wonders  revealed  by  the,  18, 
270.  What  it  is,  268.  The  single,  268. 
The  compound,  269.  Solar,  270.  Oxy- 
hydrogen,  270. 

Milky  way,  the,  400. 

Mill,  Barker's,  161. 

Mill-stones,  how  made  in  France,  148. 

Mirage,  248. 

Mirrors,  concave,  Eoman  fleet  fired  with, 
194.  What  they  are,  237.  Plane,  238. 
Concave,  238.  Convex,  238.  Eellection 
from  plane,  240.  linages  formed  by 
plane,  241.  Eeflection  from  concave, 
242.  Eeflection  from  convex,  244. 

Mississippi,  the,  its  discharge,  156. 

Mixtures,  freezing,  211. 

Mobility,  defined,  20. 

Momentum,  what  it  is,  29.  Eule  for  find- 
ing the,  80. 

Monsoons,  402. 

Montgolfar  brothers,  balloons  Invented 
by  the,  176. 


Moon,  the,  389.   Produces  tides,  157.  S!.ze 

of,  389.  Motions  of,  889.    Phases  of,  390. 

New,  390.     Gibbous,  390.    Full,  390. 

How  it  appears  through  the  telescope, 

391.    Eclipses  of,  396. 
Moons,  of  Jupiter,  392.    Of  Saturn,  393. 

Of  Uranus,  893. 
Morse,  his  telegraph,  358.  His  telegraphic 

alphabet,  361. 
Motion,  what  it  is,  27.  Absolute,  27.  Eel- 

ative,27.    Kinds  of,  28.    Uniform,  28. 

Accelerated,  29.    Eetardcd,  29.    First 

law  of,  36.    Second  law  of,  41.    Simple, 

41.  Eesultant,  41.    Parallelogram  of, 

42.  Third  law  of,  43.    Eeflected,  45; 
law  of,  46.    Eotary,  may  keep  a  body 
from  falling,  76.    Perpetual,  89.    Cir- 
cular, how  converted  into  rectilinear, 
124.    Alternate  up-and-down,  how  pro- 
duced, 124.    Eeal  and  apparent,  of  the 
planets,  880. 

Motive  Powers,  81. 
Multiplying  Glass,  the,  252. 
Muscheribroeck,  his  electric  shock,  800. 


Nadir,  the,  3S5. 

Natural  Philosophy,  defined,  9.    Modes 

of  investigation  in,  10.    Branches  of,  11. 
Nebulae,  400. 
Needles,  magnetic,  836.    Horizontal,  336. 

Dipping,  836,  341.    Astatic,  339.    How 

to  magnetize,  847.    Effects  of  electric 

currents  on  magnetic,  349. 
Neptune,  when  discovered,  375.     First 

called  Le  Terrier,  375.    Details  of  the 

planet,  394 

Nerve,  optic,  261,  262.    Acoustic,  28& 
Newcomen,  his  steam-engine,  222. 
Newton,  discovered  the  law  of  gravita- 

tion, 47.    Held  the  corpuscular  theory 

of  light,  229. 

Nimbus,  the,  described,  405. 
Non-conductors,  of  heat,  199.    Of  elec- 

tricity, 293. 
Non-electrics,  293. 
Non-luminous  bodies,  defined,  230. 
North  star,  distance  of  the,  399. 

O. 

Observation,  wHat  It  consists  in,  10. 
Occupation,  380. 


INDEX. 


447 


Ocean,  the  surface  of,  spherical,  131. 
Pressure  of,  at  great  depths,  136. 

Octaves,  what  they  are,  284. 

Oersted,  discovered  the  phenomena  of 
electro-magnetism,  349. 

Oil,  how  extracted  from  seeds,  112. 

Opaque  bodies,  defined,  231. 

Opera-glass,  the,  27& 

Opposition,  379. 

Optic,?,  defined,  229. 

Organ,  the,  284. 

Orifices,  velocity  of  streams  flowing 
through,  153.  Course  of  streams  flow- 
ing through,  154.  Volume  discharged 
from,  154. 

Oxygen,  promotes  combustion,  195.  Com- 
bines with  carbon  to  produce  animal 
heat,  196. 

P. 

Paddles,  of  a  wheel,  160. 

Pallas,  when  discovered,  3T5. 

Pallets,  of  an  escapement,  127, 129. 

Parachute,  the,  55. 

Paradoxes,  80.  Hydrostatic  Paradox,  137. 

Parallax,  395. 

Parallelogram,  defined,  35.  Of  motion,42. 

Pascal,  constructed  the  first  barometer, 

171. 
Pencil,  a,  of  light,  230.    A  diverging,  230. 

A  converging,  230. 
Pendulum,  the,  what  it  is,  65.    Laws  of 

its  vibration,  65,   66.     Application  to 

clock-work,  67.    Vibrates  differently  in 

different  latitudes,  67.    Effect  of  heat 

on  its  vibrations,  68.  Compensation,  68. 

Gridiron,  68.    Ballistic,  64.    Ita  use  in 

clock-work,  127. 
Penumbra,  the,  235. 
Percussion,  a  source   of  heat,  197.     A 

source  of  light,  232. 
Perigee,  389. 

Perihelion,  of  a  planet,  374. 
Perspective,  the  magic,  242. 
Perturbations,  394. 
Phantasmagoria,  272. 
PhHosop7iyt  natural,  9.    Meaning  of  tho 

term,  9. 

Photographic  process,  the,  233. 
Physics,  another  narno  for  Natural  Phi- 

lotfophy,  9. 
Pile,  Volta's,  318.    The  dry,  822. 


Pinions,  defined,  122. 

Pipes,  flow  of  liquids  in,  155. 

Pisa,  tower  of,  75.  Scene  of  an  interest- 
ing experiment,  54. 

Pistol,  the  electrical,  304. 

Planets,  the,  373.  Secondary,  374.  Pri- 
mary, 374.  Inferior,  374.  Superior,  874. 
Orbits  of,  374.  Table  of,  375.  Aspects 
of,  378.  Are  they  inhabited,  380. 

Plating,  326. 

Pneumatics,  defined,  165. 

Points,  the  cardinal,  341.  Of  the  com- 
pass, 342. 

Polarity,  magnetic,  838. 

Polarisation,  of  light,  252. 

Poles,  of  a  galvanic  battery,  323.  Of  nat- 
ural magnets,  334.  Of  artificial  mag- 
nets, 337.  Magnetic,  of  the  earth,  344. 
Of  the  horizon,  385. 

Pores,  what  they  are,  18. 

Porosity,  defined,  19.  Of  various  sub- 
stances, 19. 

Powers,  tho.  mechanical,  94. 

Press,  book-binder's,  115.  Bramah's  hy- 
drostatic (or  hydraulic),  138. 

Pressure,  of  liquids,  135.  Of  the  atmos- 
phere, 169. 

Prisms,  refraction  by,  248.  Decompose 
light,  255. 

Projectile,  a,  what  it  is,  60.  Forces  by 
which  it  is  acted  on,  61.  Path  of,  61. 
Random  of,  62. 

Propeller,  the  screw,  160. 

Properties,  universal,  12.    Accessory,  12. 

Ptolemy,  his  system  of  the  universe,  370. 

Pulley,  the,  106.  The  fixed,  106.  The 
movable,  107.  White's,  108.  Much  of 
its  advantage  lost  by  friction,  109. 

Pump,  the  chain,  162.  The  lifting,  186. 
The  forcing,  187.  The  centrifugal,  189. 
The  stomach,  190. 

Pupil,  the,  261.     Of  beasts  of  prey,  262. 

Pyramids,  the  most  stable  of  figures,  74 
Egyptian,  74. 

Pyrometer,  the,  215. 

Pyronomics,  defined,  192. 

Pythagoras,  the  first  to  use  the  term 
philosopJiy,  9.  Taught  the  true  theo- 
ry of  the  solar  system,  370. 


Quadrant,  defined,  85. 


448 


INDEX. 


Quadrature,  379. 
Quadrilateral,  defined,  85. 
Quarter,  first,  of  the  moon,  390.    Third, 
of  the  inoon,  390. 

B. 

Race,  a,  what  it  is,  153. 

Rack  and  pinion,  124 

Radiation,  of  heat,  201 

Radius,  defined,  34. 

Radius  Vector,  the,  377. 

Rain,  406. 

Rainbow,  the,  259.  Primary  and  second- 
ary, 260.  Lunar,  260. 

Ram,  the  hydraulic,  163. 

Random,  62.  At  what  angle  it  is  great- 
est, 63. 

Rarity,  19.    In  optics,  246. 

Rays,  what  they  are,  230.    Incident,  236. 

Reaction,  defined,  43.  Equal  to  action,  43. 
Examples  of,  43.  Often,  nullifies  ac- 
tion, 43. 

Reasoning,  by  induction,  10.    By  analo- 

gy,  10. 

Reaumur,  his  thermometrical  scale,  214 

Receivers,  177. 

Rectangle,  defined,  36. 

Reflection,  angle  of,  46.  Equal  to  angle 
of  incidence,  46.  Of  light,  236 ;  great 
law  of,  238. 

Refraction,  245.  Atmospheric,  247.  By 
convex  lenses,  250.  By  concave  lenses, 
251.  Double,  252.  Its  effect  on  the 
apparent  position  of  the  heavenly  bod- 
ies, 394. 

Refractory  substances,  defined,  210. 

Reft-igerators,  what  their  sides  are  filled 
with,  200. 

Regulator,  of  a  watch,  129. 

Repulsion,  between  the  particles  of  aeri- 
form bodies,  21.  Between  solids  and 
liquids,  147.  Electrical,  290. 

Resistance,  what  it  is,  27.  Appears  in 
various  forms,  84 

Rest,  what  it  is,  27.  Absolute,  27.  Eel- 
ative,  27. 

Restitution,  force  of,  24. 

Retina,  the,  261,  262.  Images  formed  on, 
264 

Rhodium,  one  of  the  hardest  metals,  22. 

Rivers,  velocity  of,  how  retarded,  156. 

Rocking  Horse,  the,  76. 


Rocking  Stones,  79. 

Rocks,  how  rent,  136. 

Roemer,  first  used  mercury  in  the  ther- 
mometer, 214.  Discovered  the  veloci- 
ty of  light,  234. 

Rope-dancers,  how  they  balance  thenr 
selves,  78. 

Rosse,  Earl  of,  his  telescope,  273. 

Rotation,  electro-magnetic,  353. 

Rubber,  the,  289. 

S. 

Safes,  what  the  sides  are  filled  with,  200. 

Sap,  how  it  ascends  and  descends  i» 
plants,  151. 

Saturn,  details  of  the  planet,  393. 

Savery,  his  steam-engine,  221. 

Scale,  diatonic,  285. 

Scape-wheel,  the,  of  a  watch,  127. 

Screw,  the,  what  it  consists  of,  114  Kind* 
of,  114  Advantage  gained  by,  114. 
Practical  uses  of,  116.  Hunter's,  116. 
The  endless,  117.  Archimedes',  162. 

Seasons,  the  change  of,  386. 

Sea-water,  heavier  than  fresh  water,  144 

See-saw,  the  electrical,  301. 

Selenite,  polarizes  light,  254. 

Self-luminous  bodies,  defined,  230. 

Sliadows,  235. 

Stock,  the  electric,  299;  anecdote  con- 
nected with,  300. 

Shower,  the  mercury,  182. 

Signal  key  the,  360. 

Silurus  electricus,  the,  316. 

Simoom,  the,  402. 

Siphon,  the,  185. 

Sirius,  its  light  compared  with  the  sun's, 
232.  Distance  of,  399. 

Sirocco,  the,  402. 

Sling,  the  principle  on  which  it  acts,  38. 

Smoke,  why  it  rises,  176. 

Snow,  protects  vegetation,  202.  How 
formed,  406.  Colored,  406. 

Solids,  defined,  7.  Difference  between 
them  and  fluids,  8.  Specific  gravity  of, 
142.  Porosity  of,  proved  with  the  air- 
pump,  184.  Expansion  of,  ?07.  Melt- 
ed by  heat,  210. 

Solstices,  the,  383. 

Sonorous  bodies,  defined,  275. 

Sound,  nature  of,  274.  Transmission  of, 
275.  Velocity  of,  27T.  Distance  to 


IXDEX. 


440 


which  It  Is  transmitted,  273.    Interfer-  | 
ence  of,  279.   Reflection  of,  279.    A  mu- 
sical, how  produced,  281;  loudness  of, 
2S1 ;  pitch  of,  281 ;  quality  of,  281. 

Spark,  the  electric,  297.  Color  of,  806. 
Length  of,  307.  Ignition  by,  807. 

Speaking-trumpet,  the,  279. 

Specific  gravity,  139.  Of  liquids,  141. 
Tables  of,  144.  How  to  ascertain  the 
weight  of  a  body  from  its,  145. 

Spectacles,  263. 

Spectrum,  the  solar,  255.  Properties  of 
the,  257.  Dark  lines  in  the,  253. 

Speculum,  a,  what  it  is,  287. 

Sphere,  defined,  36.  Axis  of,  36, 71.  Poles 
of,  36.  Equator  of,  36. 

Spheroid,  oblate,  36.    Prolate,  36. 

Spirit-level,  the,  135. 

Spots,  solar,  371. 

Springs,  used  as  a  motive  power,  82. 

/Springs,  origin  of,  133.  Hot,  accounted 
for,  195. 

Square,  defined,  36. 

Stability,  of  bodies,  72.  Depends  on  the 
position  of  the  centre  of  gravity,  75. 
How  increased,  76.  Of  a  sphere,  how 
increased,  79. 

Stammering,  287. 

Stars,  the,  a  source  of  light,  232.  Magni- 
tudes of,  998.  Distance  of,  399.  Peri- 
odical, 399.  Binary,  399.  Telescopic,  399. 

Staves,  of  wheels,  123. 

Steam,  the  most  eifective  of  motive  pow- 
ers, 83.  Generation  of,  216.  Tempera- 
ture of,  217.  Properties  of,  217.  Con- 
densation of,  218.  Electricity  from,  310. 

Steam-engine,  Hero's,  219.    De  Garay's, 

220.  Of  De  Caus,  220.    Branca's,  220. 
Marquis  of  Worcester's,  220.     Papin's, 

221.  Savery's,  221.    Newcomen's,  222. 
Watts',  222.   Parts  of  the,  223,  224.  The 
low  pressure,  226.    The  high  pressure, 
226.    The  locomotive,  226 ;  history  of, 
227. 

Steel,  elasticity  of,  24. 

Steelyard,  the,  97. 

Stephenson,  improved  the  locomotive,228. 

Stethoscope,  the,  principle  of,  278. 

St.  Helena,  tides  at,  157. 

Still,  the,  described,  212. 

Stilts,  used  by  French  shepherds,  78. 

Stool,  the  insulating,  298. 


Stratus,  the,  denned,  405.      . 

Strength,  of  men  and  animals,  used  as  a 
motive  power,  82.  Of  materials,  91.  Of 
rods  and  beams,  91. 

Striking  force,  81.  Difference  between 
it  and  momentum,  81.  Eule  for  find- 
ing the,  31. 

Strings,  of  musical  instruments,  282. 

Sucker,  the,  principle  of,  170. 

Sun,  the,  a  source  of  heat,  193.  A  source 
of  light,  232.  Size  of,  371.  Constitution 
of,  372.  Motions  of,  372.  Eclipses  of,  335. 

Sun-dial,  the,  126. 

Syringe,  the  fire,  197. 

System,  the  Solar,  370.  True  theory  of, 
taught  by  Pythagoras,  370 ;  revived  by 
Copernicus,  370.  '  X 

T. 

Tangent,  defined,  34. 

Teeth,  connect  wheels,  122. 

7Wefirra/>&,electro-magnetic,  358.  Morse's, 
853.  House's,  362.  Bain's,  362.  The 
sub-marine,  362.  The  Atlantic,  363. 
History  of  the,  363. 

Telescope,  the,  272.  Eefracting,  272.  As- 
tronomical, 273.  Terrestrial,  273.  Ec- 
flecting,  273.  Herschel's,  273.  Earl  of 
Eosse's,  273. 

Temperature,  what  it  is,  192. 

Tempering,  how  effected,  24. 

Tenacity,  defined,  22.  Distinguished  from 
hardness,  22.  Belongs  to  the  metals,  22. 
Of  different  substances  compared,  23. 
Of  liquids,  23. 

Thermo-electricity,  332. 

Thermometer,  the,  213.  Invention  of, 
214.  The  differential,  214. 

Thickness,  defined,  12. 

Thunder,  312. 

Thunder  house,  the,  306. 

Tides,  what  they  are,  157.  How_  pro- 
duced, 157.  Spring,  157.  Neap,  157. 
Height  of,  157. 

Tools,  defined,  88. 

Top,  why  it  does  not  fall  when  spin- 
ning, 76. 

Tornadoes,  403. 

Torpedo,  the,  816. 

Torricelli,  proved  the  pressure  of  the  at- 
mosphere, 171. 

Tourmaline,  polarizes  light,  254. 


450 


INDEX. 


Train,  of  wheels,  120.    Of  wheels  and  | 
pinions,  122. 

Transit,  of  a  planet,  880. 

Translucent  bodies,  defined,  231. 

Transparent  bodies,  defined,  231. 

Treadle,  the,  125. 

Trevithick,  constructed  the  first  practi- 
cal locomotive,  22T. 

Triangle,  defined,  35. 

Tripoli,  formed  of  fossilized  animalcules, 
18. 

Tropics,  the,  888. 

Trundle,  a,  what  it  is,  123. 

Tubes,  acoustic,  278.    Aurora,  302. 

Turbine,the,  159. 

Tympanum,  the,  288. 

Typhoons,  403. 

U. 

Uranus,  when  discovered,  875.    Its  for- 
mer names,  375.    Details  of  the  plan- 


V. 

Yacuum,  what  it  Is,  166.  Torricellian, 
172.  Fountain,  181.  Bell,  183. 

Valve,  the  safety,  226. 

Vaporization,  211. 

Vapors,  what  they  are,  165.  Conducting 
power  of,  201.  Expansion  of,  210. 

Variation,  magnetic,  340.  Lines  of  no,340. 

Velocity,  what  it  is,  27.  Rule  for  finding 
the,  28.  Of  various  moving  objects,  28. 

Ventriloquism,  287. 

Venus,  details  of  the  planet,  382. 

Verge,  of  a  watch,  129. 

Vesta,  when  discovered,  375. 

Veto,  the,  175. 

Views,  dissolving,  272. 

Vision,  260.    Defects  of,  263. 

Voice,  the  human,  285;  when  said  to 
change,  286.  Of  the  inferior  animals,2S7. 

Volta,  his  theory  respecting  galvanism, 
318.  His  pile,  31&  Invented  the  cou- 
ronne  des  tosses,  320. 

Voltaic  electricity,  816.  Effects  of,  324 
Decomposes,  325.  Luminous  effects  of, 
828.  Heating  effects  of,  329.  Physio- 
logical effects  of,  830.  Medically  ap- 
plied, 331. 

Watches,  history  of,  126.  Works  of,  128. 
How  regulated,  128.  Parts  of,  129. 


Water,  composition  of,  9.  Used  as.a  mo- 
tive power,  82.  Quantity  of,  on  the 
earth's  surface,  130.  Finds  its  level,  131. 
Conveyed  in  pipes,  132.  How  conveyed 
by  the  ancient  Eomans,  132.  Its  weight 
compared  with  air,  144.  Wheels  movetf 
by,  158.  Machines  for  raising,  161.  Ex 
pansion  of,  in  freezing,  209.  Decom- 
posed by  the  galvanic  battery,  825. 

Water-clock,  the,  126, 153. 

Water-organ,  the,  284. 

Water-spouts,  403. 

Watts,  his  steam-engine,  222. 

Wave-,  how  produced,  156.  Height  of,  157. 

Wedge,  the,  112.  Used  for  raising  weights, 
112.  Familiar  applications  of,  113.  Ad- 
vantage gained  by,  118. 

Weighing,  double,  97. 

Weight,  what  it  is,  50.  Aoove  and  below 
the  earth's  surface,  50.  Law  of,  52.  At 
different  parts  of  the  earth's  surface,  53. 

Weight-lifter,  the,  182. 

Wells,  Artesian,  133. 

Wheel  and  Axle,  the,  103.  Simply  a  re- 
volving lever,  103.  Law  of,  104.  Dif- 
ferent forms  of,  104. 

WJieels,  friction,  88.  Enter  largely  into 
machinery,  120.  Modes  of  connecting, 
120.  Different  forms  of  the  circumfer- 
ences of,  121.  Toothed,  122.  Varieties 
of  toothed,  122.  Spur,  122.  Cog,  123. 
Mill,  123,  Mortice,  123.  Crown,  123. 
Bevel,  123.  How  arranged  in  watches, 
129.  Undershot,  158.  Overshot,  153. 
Breast,  159. 

Whirlwinds,  403. 

Width,  defined,  12. 

Wind,  used  as  a  motive  power,  82.  How 
produced,  401.  Velocity  of,  how  meas- 
ured, 401.  Constant  winds,  402.  Trade 
winds,  402.  Periodical  winds,  402.  Va- 
riable winds,  403. 

Windlass,  the,  described,  105. 

Wind-mills,  SI.  . 

Worcester,  his  steam-engine,  220. 

Work,  unit  of,  84. 

Wrapping  connector,  121. 


Zenith,  the,  3S5. 
Zodiac,  the,  386. 


Signs  of,  386. 


14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

LOAN  DEPT. 

This  book  is  due  on  the  last  date  stamped  below.  Of 
on  the  date  to  which  renewed. 

re  subject  to  immediate  recall. 


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